Originally Posted by camisadelgolf
Many people were already dressed in costume. Also, he entered through an exit door.
He apparently bought a ticket. Sat in a seat for about 15 minutes or so then sometime after the lights went down he went out the exit door but somehow kept it from closing all the way. Then he went out to his car pulled it up to the back of the theater and then got his weapons and went back in through that exit door.
So that is how he got the weapons in it all came down to him going out that door keeping it open slightly somehow and then re-entering. In the theater it would have seemed unusual for someone to exit that door but with the movie starting most people probably did not pay much attention to it.