IMac (Intel) :: Moving Mail From Broken MBP To New - Permissions Error Persists

My Macbook Pro from 2007 won't boot anymore, turns out to have NVIDIA GPU problem. It ran on Snow Leopard, but I can't enter it anymore to upgrade. I'm trying to move mail to my new Imac and have a TM backup on the desktop of it. I followed this [URL]..... Mail folder and plist are now copied in the same locations, new original Mail folder is parked on desktop because I didn't dare 'fuse' the two, let alone replace it with my old mail folder.

When opening the Mail app. I get the permissions message that I should set 'read and write' permissions on the Mail folder, but it says the info Mail wants to use is in the mail folder in the Backup folder, although I provided it with a copy in its own Library. The permissions via show info were ok, but I redid them and still the error keeps popping up. 

Questions: 

Why won't it use the folder from its own library?

Could this be because my previous version is too old?

I can't enter my mail or mail addresses, years of data there..