How can i get it to take the pic? canon rebel xti?

i just got this camera, so i am not familiar with it. most of it i can work on later.. but what i mainly bought it for was to take pictures of the night sky. so i go outside tonight and try to take a bulb pic, or even a 30 sec pic, but since it doesnt think it can focus it will not even start to take the pic.. the camera was on a tripod and it was still (no camera shake) but it would not even attempt to take a pic.. what else should i do?? thanks!!!