I can't find the keyboard shortcut for this on the mac. It's F2 on a windows machine.
Excel for Mac 2016: F2
The most recent version of Office for Mac (2016) changed a number of keyboard shortcuts to create greater similarity across Windows and Mac versions of the software. Ctrl + U appears to no longer work as a shortcut key for editing cells in Excel 2016 for Mac. Instead the F2 key must be used. Depending on the keyboard settings on your particular Mac, to trigger F2 (instead of increasing screen brightness), you may have to hold down the "function" key (fn) while also hitting F2. To modify this behavior, go to System Preferences > Keyboard and check or uncheck "Use all F1, F2, etc. keys as standard function keys."
Here is the complete list: Shortcuts