but unfortunately MultiMiner doesn´t detect my GPU. Mining Scrypt Coins with Diabolo or Bitcoins with guiminer (or Bitminter) works just fine.
Any ideas? Are there any iMacs out there causing similar trouble?
MultiMiner uses either bfgminer or cgminer to work. In order to troubleshoot this you'll need to execute them directly from Terminal.app. How did you install bfgminer and cgminer? Did you install them separately or did you let MultiMiner download them? And where do you have the app (MultiMiner-XYZ.app)?