To do Alt + L A on Mac in Excel, download and install Accelerator Keys, then press Option, followed by L, A.
Command: Add-ins
This is how to do Alt + L A 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 a opens the Add-ins dialog where you can manage Excel add-ins. From here you can enable or disable add-ins, browse for new ones, Configure add-in settings to extend Excel’s functionality.