First, you have to make top and side images for the new tiles; this requires a graphics editor that can handle the BMP files that VirtualScape reads. For example, I made these images for the top and side of a 'swamp' tile to use until Zzzzz is able to add the SotM tileset to the VirtualScape default configuration by recoloring existing tile images and then saving them under new names:
These are JPEG images, so you'll need a program like the free image viewer Irfanview to convert them to BMP images. Once you have your images for the tiles, put them in the Images/Personal subdirectory of your VirtualScape folder. Now start VirtualScape and select Configuration->Personal Tiles from the menu bar. Click on 'Add', and you get a pop-up window to let you define the tile. Give it a name, which should be descriptive of the terrain type (like 'Grass', 'Rock', 'Sand', etc. -- you'll be using the same name for different hex-size tiles). Select how big the tile is -- 1-, 2-, 3-, 7-, or 24-hex -- then enter how many of that tile you have. Use the pull-down lists to pick the image to use for the top and side of each hex on the tile when it is rendered in the 3D view. Now click on the box to the right of 'Color on 2D view' to get a color-pick dialog for that tile; this is the color that shows up when you drop the tile on the 2D map. If you have several different terrain types (not just hex sizes) that use the same color, you can set the 'Character on 2D view' value so that it draws a special character on the map to let you identify it more readily in the 2D view. Now click on 'OK', and you'll see the personal tile list update with the tile you just 'created'. Now click on 'Close', and you're ready to go; if you click on the 'Personal Tiles' tab under Objects, you'll see the tile you just defined. 1.025 appears to have a minor bug in that the number of personal tiles of each type does not show up correctly, but the number really did get stored.