I't could take a while , I'm used to 3dsmax7 which I've used since version 4x, I've managed after half a day searching to import the meshes into Max, export as LWO, import into C4D export as OBJ import into 3dsmax!!
because terragen is freeware the rendering, lighting is nowhere as good as Max but now I just have to figuer out how to Map the Terrains in Terragen first and hopefully the maps cross over with the convertions to save me weeks mapping in Max.
just a lot of experimentation is needed... It scared me though and I went to college to learn how to use it, previously I was using Bryce 5 so basically I moved from a building block app to a full on modelling/animation app... Max though is easyer to pick up than it looks + theirs millions of tutorials online for all levels even the new user to get you going if you want to give it a go