Microsoft Excel Mac, keyboard shortcut to edit in cell

Excel does not offer a direct way to customize this: it is neither a menu command (which would be available to OS X’s system wide shortcuts), nor is it a shortcut configurable via Office’s dedicated Tools → Customize Keyboard… (approximate translation, I’m on a German system) dialog – as you can easily test by calling the dialog and pressing Ctrl+U in the Add Shortcut box (no assigned shortcut will be shown).

You can, however, work around the issue by remapping Cmd+Return to Ctrl+U using Takayama Fumihiko’s KeyRemap4MacBook (which, despite its name, will work on any Mac running OS X 10.4 upwards). You will have to add a private.xml configuration file to KeyRemap4MacBook like documented on the KeyRemap4MacBook site, with its XML content looking like this:

<?xml version="1.0"?>
<root>
  <item>
    <name>Remap "Edit in Cell" in Excel</name>
    <appendix>Use Command+Return instead of Ctrl+U</appendix>
    <identifier>private.app_excel_commandreturn_to_ctrlu</identifier>
    <only>EXCEL</only>
    <autogen>--KeyToKey-- KeyCode::RETURN, VK_COMMAND, KeyCode::U, ModifierFlag::CONTROL_L</autogen>
  </item>
</root>

– this will restrict the remapping to Excel (for a complete list of the keycodes used by KeyRemap4MacBook, see its source code).