My copy of FrameMaker is busy creating a PDF at the moment, so I am guessing a little here. Select the number that
is to be the exponent, open the character designer, and the choice superscript should appear in one of the tabs, not sure which tab, but it is there. If you want, you can create a character style for superscript and subscript. Again, guessing a little here. Maybe others will confirm or correct my guessing.
In addition to Van’s tip to use a superscript character format, you can adjust the scaling and position of superscripted characters using Format > Document > Text.
Another idea — if you are using FrameMaker 8 or 9 — would be to access the superscript characters which might be part of the font you are using. I collected several methods to view and access all characters of a font in a [German] blog entry, but I recommend PopChar/Win from Ergonis:
PS: My blog page is at http://cap-studio.de/wp/index.php/info/framemaker/unicode-zeichen-eingeben/
Just a note that subscripts and superscript settings are global to the document. If you want to customize the vertical positioning and size of the exponent via a character tag depending upon the font used, then Scott Prentice has an excellent plug-in available for this. See: http://www.leximation.com/tools/info/extracharformat.php