Filed and flew klas-klax. Everything went well but the last altitude clearance I got was to 13000 feet. I was never transfered to tower. And after I flew over klax airport ATC said I was off course.

Were you cleared to descend via a specific approach?

I did. I got cleared to descend via RIIVR but only got clearance to 14000, then 13000.

Joel, did you get clearance for the ILS or other approach?  If so, that is clearance to follow the altitude restrictions in the approach. 

Unless you are getting vectors, ATC will clear you for a STAR, which will give you clearance to fly those altitudes if the clearance is "Descend via the xxxx arrival".  Otherwise, they might give you step descents during the STAR and clear you to "Fly the xxxx arrival".

When you reach the approach IAF, or slightly before, you will get clearance for the ILS or other approach, which gives you clearance to descend as needed, following the altitude restrictions of the approach.

Hope this helps,

Dave

Thanks Dave, I will try again and report back.

I am happy to report that this flight was succesful! I repeated the flight today, same flight plan "KLAS.BOACH6.HEC.RIIVR2.ILS25L.KLAX". Everything went well this time. I believe the difference was the flight plan this time included the waypoint of the ILS. I do not remember them there in the previous flight plan eventhough the last waypoint said KLAX ILS 25L. Most likely my fault. This version is so much better. Great job Dave!