Did the old engine you took out run at all? If it did maybe you left something disconnected or put a connector on the wrong place. I would start with the basics, is there a spark at the plugs? Are the spark plugs getting wet, indicating that fuel is getting into the cylinder? Can you smell fuel?
It's easy to make a mistake when swapping an engine. I did it with a Ford Focus where I connected the fuel feed and return lines to the opposite unions, the result was the engine not fuelling and so wouldn't fire. Pull the plugs out and check is the best thing.
Other things that are easy to forget are the earthing cables that connect to various parts of the engine.
This is just a basic thought, I'm no BMW guru so I apologise if you've checked these areas as a matter of course.