Do you have KDE installed?
If so, try xwmconfig, select KDE, and try it then.
If that doesn't work, there is another (albeit improper) way I know of, and that includes puttting a file called .xinitrc in ~ and putting in it a line that says startkde
this is easily accomplished by entering this command (if the file's not there):
$ echo startkde >> ~/.xinitrc