Maybe ...
Sub AddTextBox()
Dim oSlide As Slide
Dim oShape As Shape
Dim MyText As String
Dim nWidth As Single
MyText = InputBox("Enter text here")
If Len(MyText) = 0 Then Exit Sub
nWidth = ActivePresentation.SlideMaster.Width
For Each oSlide In ActivePresentation.Slides
With oSlide.Shapes.AddTextBox(msoTextOrientationHorizontal, 0, 0, nWidth, 30)
.TextFrame.TextRange.Text = MyText
.TextFrame.AutoSize = ppAutoSizeShapeToFitText
End With
Next
End Sub
Also, it's easier to read if you use the VBA tag insert box
Paul