In case you came in late this tutorial starts here
3dem provides the useful service of translating our .tif file to a .dem file. You simply select File>Load Terrain Model choose GeoTiff DEM and select your file. If you can’t find your file check the folder with Explorer. You probably didn’t give it the requisite .tif extension.
Once your file opens you will see a beautiful color 3d topo of your data. If this image looks cool your data will probably make a good model. If this image is boring you might want to pick another location.
Next click File>Save as USGS ASCII DEM and you are ready for AccuTrans3d.