FIrst, do you have Excel v.X, or are you using a previous version in
Classic? If the latter, there's not a lot of flexibility.
Excel v.X's shortcut keys are nearly all customizable - just go to
Tools/Customize, click Keyboard, and find the command you want to
change. Enter a shortcut in the indicated box and click Add. If the
shortcut is already taken, you'll be given a choice to replace it or
cancel.
In addition, using OSX 10.3.3, you can use the System
Preferences/Keyboard and Mouse/Keyboard Shortcuts pane to assign
keyboard shortcuts for any application (or all applications).
There are a couple of WinXL keyboard shortcuts that aren't available in
MacXL: Toggling relative/absolute addresses is only available in the Mac
CMD-t shortcut. Entering Edit mode via F2 is not available, but see
http://www.mcgimpsey.com/macoffice/excel/f2.html
for a close workaround.