Mac Excel 2019 XLL add-in
 

Hi,
I run Excel 2019 on a Mac, and have an add-in (XLL) that I would like to use.
However, when I browse to the add-in and open it, it never appears in the add-in box ready for use.
At first I thought I could save the add-in and an excel file in the sandbox, and load the addin into the excel file, but that does not work either:
/Users/userName/Library/Group Containers/UBF8T346G9.Office
Does anyone know how to load an XLL add-in to Excel 2019 for Mac ?
Regards,
Tom