Originally Posted by JohnWilson

Is the header on the slide master?

Assuming you have a textbox on the Master with some text this macro should insert the Month and Year before the text.

Sub AddDate()
With ActivePresentation.Designs(1).SlideMaster.Shapes(" Header").TextFrame.TextRange
.Text = Mid(.Text, InStrRev(.Text, ":") + 1)
.InsertBefore (MonthName(Month(Now)) & "_") & Year(Now) & ":"
End With
End Sub