I’m extremely sorry to deliver the bad news, but you can’t just walk in, and there are no appointments. You either have to wait 'til there are tickets, or get an invitation from someone at Pixar. It is a closed studio after all…
You can get tickets when Pixar has a fundraiser, usually for the Emery-Ed Fund. Well, it happens maybe once or twice a year, but lately it’s been happening a little more. In the spring/summer it’s usually a promotion for the upcoming film, in the fall it is usually in anticipation to the DVD/Exclusive Short’s release. Tickets cost about $250 per person. You get a tour of the campus and the atrium, a screening of a movie or short, and some classes. On a quick note, sometimes Pixar invites the press, so that’s another way to get a tour.
I’m guessing the next available tickets will be in the fall in anticipation of the WALL-E DVD and BURN-E.
You could always check out the sorroundings and get a picture in front of the iconic sign…