For women, this means a long/mid-length dress, a nice cocktail dress, or a dressy pant-suit. For men, this just means wear wedding attire you feel comfortable in, whether that's a suit or dress pants and a jacket. The groomsmen will be wearing tuxedos, but guests are not expected to be in tuxes as well (Max just wanted a reason to wear a tux).
Click on the RSVP tab on the website. Search you, your date, or your kids' names as they were listed on your invitation (we mainly used peoples' full names). You should be able to select your name and RSVP on that page (including making meal selections for the reception).