Crazy Machines 2 - Jewel Digger DLC. Maybe I'll keep it in case I'll win the main game.
Yes, the keys for this DLC are working. As other keys for all DLCs for this game, so you can save them for the future, or make giveaways.
Apart from the Back to the Shop DLC, which activates the add-on, the game sees it, but Steam still says you don't have it

Here it is in Steam:
Satisfy your brains puzzle craving with even more Crazy Machines 2! Ratchet up the fun and tinker around with over 60 all-new levels! Challenge yourself with a wild new batch of elements including a monster truck, electrostatic fields, fiber optic cables, a hot air balloon and so much more.
store.steampowered.com
Activated with a key from indiegala, Steam will still say that you don't have it.
Here it is in SteamDB:
If you have SteamDB browser add-on installed, it will honestly show that you have it by marking it green.
If you check your DLCs in Steam itself, it will be there:
And it will also be in the game itself: