Allow trusted Add-ins as User instead of Admin
In order to add a Add-in (.mda) file into MS Access, the user has to be in the local admins group. Even if you are a domain admin, not having local admin rights won't work because your user profile changes when you open MS Access as Admin with other credentials.
Full Detail: In order to Install an Add-in library in MS Access 2016 (and at least the two prior versions) you have to open MS Access 'Run as Administrator', then do the install, then close and reopen as the standard user. If you are not in the local administrators group you must log off, login with admin rights, add the user into the local admins group, logout, login as user, run MS Access as Admin, install the add-in, remove the user from the local admins group, then logout and back in again to use it.
The simplest solution is to allow this whole process to be done by the regular user, as long as the appropriate settings are in the Trust Center. Could be anything; Add-in File in the Trusted Locations, Add-in File is a Trusted Document, Add-in File is Digitally Signed requirement stops install if not signed, but otherwise allow. I like the last one the best myself.
FYI Installing Add-ins in MS Access O365 is broken, you have to manually enter the registry keys yourself in the new Office365 location (very different location than Office 2016). I have submitted a bug for this, and it has been accepted