I haven't used them much but t.ed and 3d world builder seem to be good options, they save to an .x file, which you can just load in darkbasic using
load object "media\file.x",object_NO
Have you tried playing with the matrix functions in dark basic?
might be worth a go.
also try L3dT free version. use it to make your height and color maps.
load them into dark basic using the blitzwerk terrain plugin(free version)
http://forum.thegamecreators.com/?m=forum_view&t=181060&b=1
you can easily make your own terrain editor with the full version of blitz terrain.
but at least for now these tools should help you get somewhere.
kezzla
Sometimes I like to use words out of contents