First make sure that you have Pitch, Bank, and Heading hold selected on the auto pilot panel. Sim HQ did a great write up on the auto pilot, trimming and aerodynamics of the KA-50, it's a must read for newbies:
http://simhq.com/_air13/air_426a.html
In relation to trimming itself Matt Wagner produced an instructional video about trimming which can be found here: