Astroron has just beat me too it.
I was just going to state the obvious because sometime people don't realise....make sure you've taken the entire dust cap off so that the full aperture is exposed (I've seen it happen) and make sure the mirror is pointing to the sky. With a newtonian scope this means the eyepiece will be near the top of the tube.
At worst, can you take some pictures of how you're using it? It might give us a further clue as to why its not working. You should be able to see some stars even if you just point it at some random area in the sky.