Long story short, I have an extra activation code for the Skate Share Pack for anyone who bought the game used:
DTVR-8MKZ-N35L-5LX8
This is for Xbox 360, and North America (not sure if that makes a difference).
No way of knowing if it actually works, so my apologies if it doesn't for some reason. But I assume it does. I bought Skate 3 new, then sold it, then bought it again new recently. Turns out the activation code I used the first time around still works. (okay the story isn't that long)
On the card it says code expires May 11, 2011.
Activate it from the pause menu, go to "Create" and select "Activate Skate Share Pack" and then follow on-screen instructions.
Thought someone could use it. Skate Share Pack goes for $10 I think on the Marketplace.