Prophex-

2slow2matter gives good advice for how to bench run an engine. The question is, why? You bought a Nexstar because it's RTF. If you take the engine out, you're sort of defeating the purpose. There is no reason to take the engine out of the plane to break it in.

Just follow the OS break in instructions with the engine on the plane. You should only have to run one tank of fuel through on the ground before it is ready to fly. Just have someone help you restrain the plane or use some type of restraining device, because break in should be at full throttle. I just did this a few weeks ago with my brother's plane and an OS .46 LA. I think we actually ran two tanks through, just to be safe. The remaining break in can be done in the air.

Have fun!
Scott