Where is the Pak file for Call of Cthulhu

I cant find it in my ruleset folder?

Are you talking about COC 7th?
If yes, that one is in the vault, not rulesets folder, as it is encrypted.

Your FG client should download automatically from the store

I think the file should be this one, but i am not sure (based on looking at the store code, as i have it in my purchase history)
CHAFGCOCCOC7ERS.dat

If you do not see it there, make sure your FG Client has your login and it is accessing and downloading your purchases

Thanks