if you haven't been admitted yet...dress it up, you never know who you'll run into/whether your application is still under active consideration. So yeah, slacks, hell even a polo shirt, and some nice shoes will work. Don't need a tie.
If you've already been admitted...jeans and maybe a nice polo is fine, just don't wear a big oversized t-shirt with an obama mural all over it or something obnoxious. haha