W5100 ethernet shield on Mega 2560

I can't get the mega to operate while a W5100 ethernet shield is plugged in on top. Power light come on the mega board, but after compiling, there is no completion of upload.

Where do I start in troubleshooting this?

How far is the shield supposed to press down onto the mega? I can see a small gap where the cluster of 6 pins is in the centre, although they are inserted. It feels like I pressed it on as far as it will go, but I'm not sure.