Alfred,
You have to select the viewport, the viewport is not a tab and when prompted to select it, you must select it with the mouse, you cannot type in a name.
So when you are prompted, you should see your cross-hairs change to a pick box, use the pick box to select your viewport on the screen, (and if you miss, your pick box will not change to a window, or crossing window for this operation).
Is that clearer?
Let me know how it goes.
Todd