This issue has been reported by another person as well, that is a problem with Steam so unfortunately I can only make some possible recommendations:

1) Make sure you are in online mode and not playing any other games.
2) Right click on IMMURE, go to properties, then the DLC tab, make sure it is checked. It will say "Installed" to the right if it is ready.
3) Go to Library->Downloads, look for IMMURE, and bump it to the top using the upwards arrow button.
4) If all else fails, restart steam. It might not have recognized that the IMMURE DLC package has become available but restarting will usually force steam to refresh.

That is all that I can really recommend.

Doesn't work for me either, I tried all of above, tried restarting, unchecking rechecking enable DLC. The manage DLC tab says "Downloading" but it doesn't even appear in the download queue.

Similar problem. Updated the first part of the game. DLC is not installed. I've been waiting for twenty-four hours. I tried to completely delete the game, reloaded the Steam client, deleted all games except Immure, connected another Internet. Nothing helps :(

Yeah,the same is happening here

Anybody who is still having issues, try restarting your steam. It should now properly recognize the content. There was an issue that we had to correct with steam to make the DLC properly available. Sorry about any confusion!