So the original backtrader is officially not maintained anymore.
@heywhat host is the host IB is running on (127.0.0.1 if you run IB & BT on the same computer). Port is "socket port" in the picture in the link you just gave. Usually, 7496 for live (real) trading, and 7497 for paper trading but check on TWS if this is really the case on your platform. ClientId is your choice, this parameter exist to be able to connect different programs to IB. Please note that clientid 0 is a special kind of API connection (master one, see IB API's doc)