You cannot select anything in Presentation Mode
Try this code and give the SHAPE with the text an action of run Macro then click on the shape in show mode.
Code:
Sub trans_text(oshp As Shape)
Dim otxr2 As TextRange2
Dim L As Long
If oshp.HasTextFrame Then
Set otxr2 = oshp.TextFrame2.TextRange
For L = 1 To otxr2.Characters.Count
otxr2.Characters(L).Font.Fill.Transparency = Rnd * 1
Next L
End If
End Sub