As a rule all files in the "At startup" folder are opened by Excel at
startup, as long as they do not have the system or hidden file attributes
set. So I think your add-in should be opening. (Since add-ins are not
visible how do you know it's not opening?)

Btw, putting add-ins in the "At startup.." folder is okay but typically this
technique is not used as often as (1) the XLSTART folder, or (2) using
Tools, Add-ins to auto-load the add-in.

--
Jim Rech
Excel MVP