I went last year at Atlanta. You set up a time to meet with United online through the OBAP site. Show up at the convention center well before that time because you'll probably have a line to get your convention packet and pass. When you go in get in line about 30 minutes prior to your allotted time. At Atlanta they were running a bit behind so they asked what your time was and told you when to come back.
Wear a suit like you're going to an interview. Bring a resume with the correct airline name on it.