Hallway judging is open Friday and part of Saturday. They are judging based on category and skill level, and there are also categories like kids and group and closet cosplay. Basically if you made most of it yourself you qualify for hall judging.
Here's how they categorized hall judging last year: http://www.acen.org/forums/index.php...-contest-2012/
You fill out an entry form and fill out your name, phone number (they call you on Saturday afternoon if you won an award), and important info you want them to know about your costume. It also helps if you bring reference photos of your character/version and some progress photos. They'll talk to you for about ten minutes or so and ask questions about how you made it.
I was nervous my first time entering but it's a very relaxed setting, though sometimes they are pressed for time and have to rush through interviews. It's a great first step to get a feel for competitions.