To use Couchpotato with NZBFinder please go to Settings in Couchpotato and select Searchers. Scroll down to Newznab and make it look like this (enter your own API keys):

Couchpotato and NZBFinderCouchpotato

We personally like to give releases found via NZBFinder a little higher score. If you want that too you can add a number higher than 0 in the Extra Score box.