Board Thread:General Discussion/@comment-26184337-20160812175900/@comment-679780-20160812200644

If you go to your game library and clock "Sonic & All-Stars Racing Transformed" on your list, should show the steam library page on it.

DLCs available should be under achievements as well as a "Find more DLC in stores" link.

Though checking, don't know what you mean by this PC DLC, the only available ones I can find are Metal Sonic, Yogscast and Ryo Hazuki.