Not a whole lot to go by here without knowing which settings you have tried - I’ll give it a go though (using Radarr myself atm).
Port: Unless you’ve changed anything, I think the default is 5050
API Key: Must be generated via CP (General Settings with advanced views enabled if I remember correctly).
CP Base URL: Can’t remember, might just be /couchpotato
User/pass: Which ever login you picked for CP upon setup.
If non of this helps you any further let me know - I can install CP on my box later to test hands on with Ombi