To do Alt + L U on Mac in Excel, download and install Accelerator Keys, then press Option, followed by L, U.
Command: Use Relative References
This is how to do Alt + L U on Mac in Excel with Accelerator Keys, the native ribbon and alt key shortcut solution for Mac users who get more done with Excel.
Alt + L u toggles the Use Relative References setting for macro recording. When enabled, recorded macros use relative cell references instead of absolute references, making the macro work relative to the active cell rather than specific cell addresses.