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I've already purchased the DLC, but it's showing up for me on the store under "Owned". The DLC does not appear on Steam's DLC page, it is all done through the game. You need to buy funds using your Steam Wallet and then spend those in game.
 
It's working okay over here. I can still see the DLC that is for sale, as well as the free ones that I've already gotten.

Do like LittleMikey said and verify your games files through Steam, also make sure that the game isn't blocked by your firewall and or antivirus program or something along those lines.
 
Restarting Steam is the #1 cure for all sorts of problems. :) Glad you got it to work!