1. There should be a code on the boarding pass that will allow you to select from the Buy On Board (BOB).
2. I think you get into all MLL's on the way. I'm curious though, why did you buy this when as a UA 1K you are entitled to free access anyways?
Edit: According to this
link the MLL access only applies to the departing city. I don't know if "departing city" means original departing city, or each departing flight.