You are right that does produce an error because the argument requires the relative page position in the Tab control's Pages collection.
Assuming that you have a three page tab control called MyTabCtl with pages named Page01, Page02 and Page03. These will have a Tab Collection reference of 0, 1 and 2. This is because Collections always number from zero.
So for the above example set up three buttons and use the following code.
Private Sub cmdbtn1_Click()
Me.MyTabCtl.Pages(0).SetFocus
End Sub
Private Sub cmdbtn2_Click()
Me.MyTabCtl.Pages(1).SetFocus
End Sub
Private Sub cmdbtn3_Click()
Me.MyTabCtl.Pages(2).SetFocus
End Sub
I have tested it and it does work. Really.
Rod