Make an folder named nxtgn in your custom_plugins folder which is is located within your config folder. From CouchPotato goto Settings->About->Directories (second folder is your config folder) for location. Place the init.py and main.py files inside the nxtgn folder.
Restart CouchPotato. Then go to Settings->Searcher to configure just like any other provider. If nxtgn doesn't appear as an option, refresh your browser using CTRL + F5 as you may be seeing a cached version.