As far as I know .rlf files can only be opened with ArtCam. Did a search and found this "You can convert RLF file to STL in ArtCam. Go to relief menu->create triangle mesh, create triangles->save triangles in STL."
Gave it a go with ArtCam2008 and one of the free models from the link you provided and was able to open the resulting .stl in mastercamx3 no prob. So you will need ArtCam if you want to use the files, not sure what the Demo can do for you.
Hope this helps