The Business Class lounge is up an escalator to the left behind the brainwashed teddy bear.
Al Mourjan is above the security area for arrivals at A gates, D gates, even numbered C gates and buses from that side of the airport.
There is a separate security, under Al Safwa, for B gates, E gates, odd numbered C gates and buses from the seaward side of the airport. The train may change this since each train stops at one of the security areas depending upon which side of the airport it is on but you sometimes can chose sides.