If you are missing a game from your Dimm drive list, chances are it's quite easy to recover. Try the below 2 options

Option 1
1. Open Dimmdrive
2. Click Settings -> Reset steam config
3. Close both steam and dimm drive.
4. Open dimm drive again and check to see if your game is present

Option 2 - If Option 1 fails.
1. Open "C:\Program Files (x86)\Steam\SteamApps\common" in windows explorer.
2. Browse to the game that is missing. The game will have ".dd.postSync" and a ".dd.preSync" at the end of the folder name.
3. Remove the ".dd.presync" from the folder name.
4. Close and re-open Dimm Drive and your game will appear.

I'd take it this happens if dimm drive does not close down correctly.
The above can probably be solved by refreshing your steam cache but that could involve a lengthy download.

I haven't seen the above (At least Option 2) anywhere else so let me know if this is a known resolution.

PS, Sorry about the crappy guide image....