UPDATE 16/8/13
Thanks to EA making many online passes now free, the skate share pack can now be downloaded for free from
http://marketplace.xbox.com/en-US/Product/Skate-Share-Pack/0...So now you can get the achievement even if you didn't have a code for the pack, as now everyone can download it for free!
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UPDATE 1/7/12
DillonDay has said that the uploading process doesn't work on the 4gb memory unit you get with the Xbox 360 slim - if you want this achievement, make sure that you're using either a non-slim Xbox or if you're using a slim Xbox make sure it's the 250gb one. Try it round a friend's house if you've got a slim 4gb Xbox.
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UPDATE 23/5/12
I think we can now safely say that you need a code from the box, but now it will work when you put in your code. Thanks to 'FuhzDHAT' for confirming that its not on for everyone and thanks to 'young tugboatz' for confirming that it now works when you put in your code.
So, if you bought the game new you can get this for free, otherwise you'll probably hae to pay.
Original solution:
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Artsy Fartsy - upload 5 films, 5 photos and 3 skate. parks
Not to detract from the other solutions, as they give you a good idea of how to get the achievement, but this solution is mainly to inform people about the obtaining of a skate. share pack
I purchased my copy of the game new, but when I entered my code, it wouldn't allow me to download the pack, due to the code being expired. However, when I went on the game more recently, I found that, when looking in the dlc menu, there was a tick in the skate. share pack box. I tried uploading the videos, photos and parks (and rating other people's for the constructive criticism achievement), and voila, achievement unlocked - Artsy Fartsy.
I have therefore come to the conclusion that either EA has returned the feature to redeem codes, OR that they have turned the skate. share pack on for everyone. Please post in the comments if this works for you and it lets you redeem your code to get this achievement ticked off.
Thanks for reading, hope I helped you out!