How did you migrate over to the new computer? Did you copy the entire add-ons folder?
Are you missing just a few stacks or every 3rd party stack?
If you didn’t copy the entire addons folder (by default it’s stored in the hidden directory folder) then that’s what you need to do. Go to the backup or old Mac and find the addons folder and copy it over.
If you are just missing a few stacks then there’s supposed to be a way to find that out, but apparently there’s a bug in stacks and it’s not working. Since we haven’t heard much from @isaiah in quite a while, don’t have a clue as to when it might get fixed.
As far as I understand the Rosetta thing that Rob @Rovertek mentioned, that effect’s only older plugins, not individual stacks. Don’t know for sure either as I don’t own (probably never will) a Mac with a non-Intel processor.