There are various ways of doing this. For example:
Code:
ActiveDocument.Tables(1).Cell(5, 5).Formula Formula:="=A2/B2*100 \# ""0.00"""
Code:
With ActiveDocument
.Fields.Add Range:=.Tables(1).Cell(5, 5).Range, Type:=wdFieldEmpty, Text:="=A2/B2*100 \# 0.00%", PreserveFormatting:=False
End With