I found it on linuxpackages, with matching epplets, utils and docs too.
Here's what I get by typing "whereis enlightenment"
enlightenment: /usr/X11R6/bin/enlightenment /usr/bin/X11/enlightenment /usr/X11/bin/enlightenment /usr/X11/man/man1/enlightenment.1.gz
I can't see anything like "usr/share/apps/kdm/sessions/enlightenment.desktop" as you said.. actually the folder "usr/share/apps" doesn't exist. so...which file do I have to edit? I know I have just to add a line with the enlightement path, but I don't know where...