too late to help you with your question, but for others, you have heard about you can only make first impressions just once......dress with that in mind and you cant go wrong.
I went to NYCOM's open house and I wore some khaki pants, suede shoes, nice button down shirt and a sports coat. I saw A LOT of people with jeans. I don't think anyone looked over dressed or under dressed really. I would suggest business casual though and skip the jeans and sneakers for 5 hours.