On the PC, you can edit the current cell in place by simply pressing F2. (You could do it the other way around, of course, setting your PC up to match your Mac.) This works well for most shortcuts, but there’s one in particular that I use all the time that’s immune to this solution: the keyboard shortcut for ‘edit this cell.’
Using the View -> Customize Toolbars & Menus menu item (in Excel 2008 View -> Toolbars -> Customize Toolbars/Menus in Excel 2004), you can change the shortcuts on your Mac to match those on your PC.
If you use Excel on both the Mac and the PC, you’re probably aware that there are numerous differences between the two platforms’ keyboard shortcuts.