Very nice of you, but would you please stop giving me these newbie advices??? That is why I said no-newbie in the title... I shure know what Unit Groups are and how to work them, however I have Object Groups in my map. As I said, you can use Object Groups as Unit Groups that are already defined when the game starts. If you don't know what Object Groups are check out this tutorial. Now, in a map with just one Object Group, a simple "Pick each unit in (MyObjectGroup)" and "Kill picked unit" works just fine.
My question again; Is there a setting or anything else that prevents the usage on Object Groups as Unit Groups?