Hmmm...realised I misread your post. "Designing" an imposition. i.e.; at the stage of using the Numbering Tool? Thought you were at the stage of positioning page numbers to print outside the bleed area. Tried Preferences Trash/reset? Is there anything under the Help menu, or in the online Manual? Can't help till Monday, sorry.
If you can bother/are desperate enough, you could join/post, here:
Preps - GUA Forums
be aware; getting on and signed up is sl-o-o-w...days/weeks. Might help for next time, though.
You could cruise their old posts, meantime.