I used this program too you just need to export the heightmap when you are at that step and just go on export --> active map now save it as .bmp.

Now open med go on File --> Import --> Import Terrain from Picture and search your file where you saved it and open it. It should open a window where you can adjust the size of the map then press ok and there is your map.

Now go back to L3DT klick next until you it has finished the skin of the terrain and go again on export --> active map now save it as .bmp. Now you only have to open the skin editor in med and open the skin where you saved it again and now you have your fully finished terrain in med. Just save it as .hmp and open it in wed. Thats all I hope that helped.