I think you're out of luck.
You would have to programmatically set the macro security of the user's application, which would be defeating the purpose if that were possible. Then you would have to browse to the add-in, and checkmark it...by code...which may be doable, but a giant PITA to code.
You could create an installer program (.exe) to put the add-in where it belongs (presumably the add-ins folder), and then tell your users to go to [File]-[Options]-[Add-ins]-......Etc, you know how to get to add-ins, I'm sure. But again, a PITA.
I would probably send/give the add-in file to my users, explain to them how to place it into the proper location, and then explain to them how to make it load automatically (after they browse for it initially, of course) and call my part of the bargain finished. If they need to use it bad enough, they'll figure it out...or ask you to further explain.
All that is, of course, assuming you're sending this to other people and wanted to make it easier for them.
There's a limit to how much trouble I'm willing to put MYSELF through in order to save the End User 5 minutes of reading and following instructions.