| I'm going to assume you want to use the terrain as a 3D object... so what you'll want to do is export out your terrain as a 3ds file, or something similar. You can do that by using the File menu, and choosing Export Object (funnily enough). Try it at 256x256 first, and then turn up the detail later if you feel you have to.|
If you have the tools you can convert your terrain into another format, like the b3d one... if you have to for some reason. Make sure when you export the file out, you don't crank the polycount too high.