I have encountered the Q1 ALL IN PARK DLC package that I recently purchased. While I can confirm that the purchase has been successfully made, I have noticed that two of the contents in the package are not showing up in the game. In an attempt to resolve this issue, I tried accessing the Steam Workshop content page for the DLC, but I received the following error message: "The item is either marked as hidden or you do not have permission to view it."
That error message is due to changes Steam made quite some time ago. S397 was in the process of adding store functionality to the main game so it was kinda ignored. What you can do now is load rF2 and look for PENDING UPDATES at the top of the main menu. That should start installing any missing content. You can also try verifying files in STEAM before launching rF2.