Missing 'Manage Plug In' button in Apple Mail

I’m trying to install a mail plug in (MsgFiler) and part of the install process requires me to go to Preferences>General and select “Manage Plug Ins” button but it’s not there.

I checked the Apple Support forum and there are others with this issue but no solution - has anyone come across this and found a solution?

Part of the install process does require that an install of a ‘Bundles’ folder in Library/Mail folder and then to run to functions in Terminal (which I’m not experienced with) if one hasn’t been used before (there wasn’t) I copy/pasted the provided function but I don’t know if that worked. I’m not sure if I did something out of order/did it wrong or if there is a way to reset and start over.

Any help would be appreciated - thanks!

I think this will fix you up.

https://www.macupdate.com/app/mac/58147/mail-plugin-manager

Nope. The app is no longer sold by that developer and resolves to a 404 error. You can see what the page originally looked like through archive.org, though. And the Macupdate download link similarly sends you to an error page.