I would recommend two things:
1. Look at the ball as part of the system, contributing to the angular momentum. If you consider the ball immediately before and immediately after the collision, you will be able to view it its motion as approximately rotational, just as the motion of the free end of the bar is initially going to be approximately linear.
2. Your conservation of angular momentum equation is, I think, the way to go. You'll have the following:
Does that give you a nudge? What's your target variable?