I'm using the airports EDDM (standard), EDDL (Just sim) and EDDK (standard). How can I enable the bus?
The bus should always be enabled, unless you (or the scenery developer, if they supplied a GSX customization file), explicitly disabled it from the GSX parking customization.
Are you SURE you were on a parking that required a bus. In the default scenery, jetways won't appear if your Scenery Complexity is not high enough so, you might not "see" the jetway, but is still there, so it won't call a bus. However, you should have seen GSX L2 replacement jetways instead, at least at EDDM.
Not seeing the bus at default EDDM seems strange, since it should always appear, even if both GSX and GSX Level 2 are still in Trial, because it IS a Trial airport.