Alright, I changed your code just a little, I commented out the Call
DeleteIfExisting, because you did not include that function, and I
changed ActiveWorkbook, to ActiveDocument since we are working inside
of Word.  Also, I could tell that you were attempting to modify the
function to get the values into the function, I changed that back.

When you call functions which have values, you have to provide the
information into the function.  For instance, in the function you
provided, each of the values requested by the function are listed
between the parens.  The function also tells you what type of value it
is expecting.  For instance the strPropName as String tells you that
you should provide a name and since it is a string value, you should
enclose the name in quotes.

Sub addCustomProp()
'This can be run from the Macros window.
Call AddCustomDocumentProperty("CustomDate", msoPropertyTypeDate,
"02/28/06", False, False)
End Sub

The hard part that you have is that you will have to know what your
valid msoProperty types are.  They are limited:

msoPropertyTypeNumber - used for numbers, not decimal

msoPropertyTypeString - used for text

msoPropertyTypeDate - used for dates

msoPropertyTypeBoolean - used for true / false, yes/no

msoPropertyTypeFloat - used for decimal numbers or extremely large
numbers, I can't think of a reason to use this property type.

Function AddCustomDocumentProperty(strPropName As String, _
                                lngPropType As Long, _
                                Optional varPropValue As Variant =
"", _
                                Optional blnLinkToContent As Boolean
= False, _
                                Optional varLinkSource As Variant =
"") _
                                As Long

  ' This procedure adds the custom property specified in the
strPropName
  ' argument. If the blnLinkToContent argument is True, the custom
  ' property is linked to the location specified by varLinkSource.
  ' The procedure first checks for missing or inconsistent input
parameters.
  ' For example, a value must be provided unless the property is
linked, and
  ' when you are using linked properties, the source of the link must
be provided.

  Dim prpDocProp As DocumentProperty

  ' Validate data supplied in arguments to this procedure.
  If blnLinkToContent = False And Len(varPropValue) = 0 Then
     ' No value supplied for custom property.
     AddCustomDocumentProperty = ERR_CUSTOM_LINKTOCONTENT_VALUE
     Exit Function
  ElseIf blnLinkToContent = True And Len(varLinkSource) = 0 Then
     ' No source provided for LinkToContent scenario.
     AddCustomDocumentProperty = ERR_CUSTOM_LINKTOCONTENT_LINKSOURCE
     Exit Function
  ElseIf lngPropType < msoPropertyTypeNumber Or _
        lngPropType > msoPropertyTypeFloat Then
     ' Invalid value for data type specifier. Must be one of the
     ' msoDocProperties enumerated constants.
     AddCustomDocumentProperty = ERR_CUSTOM_INVALID_DATATYPE
     Exit Function
  ElseIf Len(strPropName) = 0 Then
     ' No name supplied for new custom property.
     AddCustomDocumentProperty = ERR_CUSTOM_INVALID_PROPNAME
     Exit Function
  End If

'   Call DeleteIfExisting(strPropName)

  Select Case blnLinkToContent
     Case True
        Set prpDocProp = ActiveDocument.CustomDocumentProperties _
           .add(Name:=strPropName, LinkToContent:=blnLinkToContent, _
           Type:=lngPropType, LinkSource:=varLinkSource)
           ActiveWorkbook.Save
     Case False
        Set prpDocProp = ActiveDocument.CustomDocumentProperties. _
           add(Name:=strPropName, LinkToContent:=blnLinkToContent, _
           Type:=lngPropType, Value:=varPropValue)
  End Select
End Function

To call it and provide the name of the property, the type of the
property, the value of the property and whether the property is linked
to content:

Sub addCustomProp()
'This can be run from the Macros window.
Call AddCustomDocumentProperty("CustomDate", msoPropertyTypeDate,
"02/28/06", False)
End Sub

The hard part that you have is that you will have to know what your
valid msoProperty types are.  They are limited:

msoPropertyTypeNumber - used for simple numbers, not decimal

msoPropertyTypeString - used for text

msoPropertyTypeDate - used for dates

msoPropertyTypeBoolean - used for true / false, yes/no

msoPropertyTypeFloat - used for decimal numbers or extremely large
numbers, I can't think of a reason to use this property type.