You must beat all film, photo, contests, own the spot, pro, s.k.a.t.e challenges.
There are 13 total s.k.a.t.e challenges, each unlocked by beating the one before it. The easiest way to beat them is doing 540s, as the AI can never copy them. Multiple kick or heelflips also do the trick sometimes, but are much more difficult to pull off and the AI can sometimes copy them.
Some of the challenges are difficult and require practice, just be patient and you will get it done.