This is Microsoft's official answer for Mac user's with a notebook:
- Quit Excel
- Go to UserName:Library:Preferences*
- Delete the file: com.microsoft.Excel.plist
* Username:Library means look for the Library folder in your user account. To see the Library in 10.7 and 10.8 you must open the Finder and then hold the Option key while looking under the Go menu in the Finder.
If you seriously miss the extended FN keys of the standard wired keyboard you might find Keyboard Maestro (a macro creation program) useful. Among other things, I have keyboard shortcuts to emulate the FN keys so I don't have to hit FN which I find annoying. And it lets me create FN equivalents for the keys I'm missing on my MBP.