You use a modelling package, that lets you create and dynamically edit the verticy and face data of a model to create a model of whatever you want, be it a house, a person, a gun, whatever. Do a google search for Wings 3D, as its a good free modeller. If you become pretty good with Wings, you might want to move onto a student version of 3DS Max. Then again if your only intention is modelling worlds and levels, you might want to look into Cartography Shop, a professional world editor.
DBP Primitive objects are usually only meant to be used as placeholders, until you create your external 3D media with modelling applications and import the media using the DBP Loading commands.
Modelling applications and World Editors are 2 completely different things. As I just said, modelling apps let you actually create the media for your world, like a factory building, a museum building, a pot for a plant, whatever you can think of.
World Editors let you load in all of your 3D media and move them around, and then export the entire world as a single mesh. You can, fcourse, do this in the modelling app as well. The main reason people use World Editors for placing down their models is because they can program their own world editor to allow them to add things like spawn points, particle emitters, lights, etc.