Admissions events are typically called info sessions. And it is these that I am referring to (just in case the admissions even you're referring to is different). You shouldn't be nervous about it because you shouldn't be trying to make an impression. There is a very slim chance they will remember you. Even if they do, it will not be of any added benefit for your application anyway.
The info session is just that - a forum for you to (1) ask any question you might have that hasn't been answered by the website and (2) meet some alumni to ask them some specific questions. If you don't have a specific question to ask - I don't think its even worth going. Rather spend the time on GMATs / essays.
The worst thing you can do is worry too much about "sounding smart" and trying to make a positive impression. You might just end up trying to say something or ask about something that was already covered in the presentation (if there is one).