Please do the following:
- Make sure that you're online, as the game may be trying to confirm ownership with the app market before enabling the DLC.
- If you're playing on Steam, please also make sure that Steam is in Online Mode (which is different from whether the computer is online).
- If you had to make any changes to your online status per the above bullet, restart your computer/device and relaunch the app to ensure that everything starts up while in an online state.
- Make sure that you're looking at the right things to evaluate whether the game is recognizing the DLC.