You may have to make a session entry for your DM to recognize it.Copy a session file from /usr/share/xsessions/, modify it to your needs, then put it in the /usr/share/xsessions directory (Renamed, of course.) Although I have no idea why anyone would want to force themselves to use TWM-- it's like grating nails on a chalk-board...
If you want something more serious, xmonad is a million times better than TWM.
