Here are a couple of ways:

' a Macro to insert cardtext for numbers up to 999,999.999,999
' Macro created 29/09/99 by Doug Robbins
   With Selection.Find
       .Text = ","
       .Replacement.Text = ""
       .Forward = True
       .Wrap = wdFindStop
   End With
   Selection.Find.Execute Replace:=wdReplaceAll
numtext$ = Int(Val(Selection.Text))
If Val(numtext$) < 1000000 Then
   Selection.Delete
   Selection.Fields.Add Range:=Selection.Range, Type:=wdFieldEmpty, _
       PreserveFormatting:=False
   Selection.TypeText Text:="=" & Val(numtext$) & " \* CardText"
   ActiveWindow.View.ShowFieldCodes = Not ActiveWindow.View.ShowFieldCodes
   ActiveWindow.View.ShowFieldCodes = Not ActiveWindow.View.ShowFieldCodes
   Selection.Fields.Update
ElseIf Len(numtext$) < 10 Then
   millions = Val(Left(numtext$, Len(numtext$) - 6))
   Balance = Val(Right(numtext$, Len(numtext$) - Len(millions)))
   Selection.Delete
   Selection.Fields.Add Range:=Selection.Range, Type:=wdFieldEmpty, _
       PreserveFormatting:=False
   Selection.TypeText Text:="=" & millions & " \* CardText"
   Selection.MoveRight Unit:=wdCharacter, Count:=2
   Selection.TypeText Text:=" million "
   Selection.Collapse Direction:=wdCollapseEnd
   If Balance <> 0 Then
       Selection.Fields.Add Range:=Selection.Range, Type:=wdFieldEmpty, _
       PreserveFormatting:=False
       Selection.TypeText Text:="=" & Balance & " \* CardText"
   End If
   ActiveWindow.View.ShowFieldCodes = Not ActiveWindow.View.ShowFieldCodes
   ActiveWindow.View.ShowFieldCodes = Not ActiveWindow.View.ShowFieldCodes
   ActiveDocument.Fields.Update
ElseIf Len(numtext$) < 14 Then
   billions = Val(Left(numtext$, Len(numtext$) - 9))
   millions = Val(Mid(numtext$, Len(billions) + 1, 3))
   Balance = Val(Right(numtext$, Len(numtext$) - Len(millions) -
Len(billions)))
   Selection.Delete
   Selection.Fields.Add Range:=Selection.Range, Type:=wdFieldEmpty, _
       PreserveFormatting:=False
   Selection.TypeText Text:="=" & billions & " \* CardText"
   Selection.MoveRight Unit:=wdCharacter, Count:=2
   Selection.TypeText Text:=" billion "
   Selection.Collapse Direction:=wdCollapseEnd
   If millions <> 0 Then
       Selection.Fields.Add Range:=Selection.Range, Type:=wdFieldEmpty, _
       PreserveFormatting:=False
       Selection.TypeText Text:="=" & millions & " \* CardText"
       Selection.MoveRight Unit:=wdCharacter, Count:=2
       Selection.TypeText Text:=" million "
       Selection.Collapse Direction:=wdCollapseEnd
   End If
   If Balance <> 0 Then
       Selection.Fields.Add Range:=Selection.Range, Type:=wdFieldEmpty, _
       PreserveFormatting:=False
       Selection.TypeText Text:="=" & Balance & " \* CardText"
   End If
   ActiveWindow.View.ShowFieldCodes = Not ActiveWindow.View.ShowFieldCodes
   ActiveWindow.View.ShowFieldCodes = Not ActiveWindow.View.ShowFieldCodes
   ActiveDocument.Fields.Update
End If

and another

Function ConvertCurrencyToEnglish(ByVal MyNumber)
Dim Temp
Dim Dollars, Cents
Dim DecimalPlace, Count

ReDim Place(9) As String
Place(2) = " Thousand "
Place(3) = " Million "
Place(4) = " Billion "
Place(5) = " Trillion "

MyNumber = Trim(Str(MyNumber))

DecimalPlace = InStr(MyNumber, ".")
If DecimalPlace > 0 Then
   Temp = Left(Mid(MyNumber, DecimalPlace + 1) & "00", 2)
   Cents = ConvertTens(Temp)
   MyNumber = Trim(Left(MyNumber, DecimalPlace - 1))
End If
Count = 1
Do While MyNumber <> ""
   'convert last 3 digits to English Dollars
   Temp = ConvertHundreds(Right(MyNumber, 3))
   If Temp <> "" Then Dollars = Temp & Place(Count) & Dollars
   If Len(MyNumber) > 3 Then
       'remove last 3 comverted digits
       MyNumber = Left(MyNumber, Len(MyNumber) - 3)
   Else
       MyNumber = ""
   End If
   Count = Count + 1
Loop

'clean up dollars
Select Case Dollars
   Case ""
       Dollars = "NoDollars"
   Case "One"
       Dollars = "One Dollar"
   Case Else
       Dollars = Dollars & " Dollars"
End Select

'clean up cents
Select Case Cents
   Case ""
       Cents = " And No Cents"
   Case "One"
       Cents = " And One Cent"
   Case Else
       Cents = " And " & Cents & " Cents"
End Select
ConvertCurrencyToEnglish = Dollars & Cents
End Function
=================================================
Private Function ConvertHundreds(ByVal MyNumber)
Dim Result As String
If Val(MyNumber) = 0 Then Exit Function

'append leading zeros to number
MyNumber = Right("000" & MyNumber, 3)

'do we have hundreds place digit to convert?
If Left(MyNumber, 1) <> "0" Then
   Result = ConvertDigit(Left(MyNumber, 1)) & " Hundred "
End If

'do we have tens place digit to convert?
If Mid(MyNumber, 2, 1) <> "0" Then
   Result = Result & ConvertTens(Mid(MyNumber, 2))
Else
   'if not, then convert the ones place digit
   Result = Result & ConvertDigit(Mid(MyNumber, 3))
End If
ConvertHundreds = Trim(Result)
End Function
======================================================
Private Function ConvertTens(ByVal MyTens)
Dim Result As String
'is value between 10 and 19?
If Val(Left(MyTens, 1)) = 1 Then
   Select Case Val(MyTens)
       Case 10: Result = "Ten"
       Case 11: Result = "Eleven"
       Case 12: Result = "Twelve"
       Case 13: Result = "Thirteen"
       Case 14: Result = "Fourteen"
       Case 15: Result = "Fifteen"
       Case 16: Result = "Sixteen"
       Case 17: Result = "Seventeen"
       Case 18: Result = "Eighteen"
       Case 19: Result = "Nineteen"
       Case Else
   End Select
Else
   Select Case Val(Left(MyTens, 1))
       Case 2: Result = "Twenty "
       Case 3: Result = "Thirty "
       Case 4: Result = "Forty "
       Case 5: Result = "Fifty "
       Case 6: Result = "Sixty "
       Case 7: Result = "Seventy "
       Case 8: Result = "Eighty "
       Case 9: Result = "Ninety "
       Case Else
   End Select

   'convert ones place digit
   Result = Result & ConvertDigit(Right(MyTens, 1))
End If
ConvertTens = Result
End Function
======================================================
Private Function ConvertDigit(ByVal MyDigit)
Select Case Val(MyDigit)
   Case 1: ConvertDigit = "One"
   Case 2: ConvertDigit = "Two"
   Case 3: ConvertDigit = "Three"
   Case 4: ConvertDigit = "Four"
   Case 5: ConvertDigit = "Five"
   Case 6: ConvertDigit = "Six"
   Case 7: ConvertDigit = "Seven"
   Case 8: ConvertDigit = "Eight"
   Case 9: ConvertDigit = "Nine"
   Case Else: ConvertDigit = ""
End Select
End Function
===========================================================
Sub TestAboveFunction()
Dim MyNumber
MyNumber = 551521896.32
MsgBox ConvertCurrencyToEnglish(ByVal MyNumber)
End Sub
===========================================================

See the section 'Formatting cash anounts in words' -
http://www.gmayor.com/formatting_word_fields.htm