Hi JTB - one possibility would be to make a grayscale image in something like Photoshop or PaintShop Pro, and then use the ZSurf utility to make a surface from it. It will do stuff like make a hill where you put white, and the ground level will be black so if you put in gradients and blends that can make a terrain. See here for info on ZSurf: http://moi3d.com/forum/messages.php?webtag=MOI&msg=768.1

Another possibility would be to create a planar surface that has many control points in it, you can then turn on points and move them up or down to sculpt a terrain shape. You can use Extrude with the "set path" option to build the base surface for this, see here for details: http://moi3d.com/forum/index.php?webtag=MOI&msg=817.4

Here is an example of the second method - here I selected different points and moved them up, then grabbed a smaller set along the edge and tweaked them a little down, etc..

I think the ZSurf method may be a little easier for you to get more precise control over it though.