The basic idea is explained at
http://www.word.mvps.org/FAQs/TblsFldsFms/ValidateFFields.htm. The
specific test in the example macro won't do, of course. The code you
need looks something like this:

Sub ExitText1()
   With ActiveDocument.FormFields("Text1")
       If Len(.Result) = 0 And _
               ActiveDocument.FormFields("Check1") _
               .CheckBox.Value = True Then
           Application.OnTime _
               When:=Now + TimeValue("00:00:01"), _
               Name:="GoBacktoText1"
           MsgBox "You must complete this field"
       End If
   End With
End Sub

Sub GoBacktoText1()
   ActiveDocument.Bookmarks("Text1").Range _
       .Fields(1).Result.Select
End Sub

--
Regards,
Jay Freedman
Microsoft Word MVP        FAQ: http://word.mvps.org