Hey, I did it.
(Note, this was all as root)
go to /etc/X11/wmsession.d/
Make an new file flux, call it FluxBox, and have this in it:
DESC="Flux baby yeah"
Changing the DESC to whatever you want
Then, to edit GDM go to /etc/X11/gdm/Sessions
And make a new file for Flux, call it FluxBox and have it contain:
exec /etc/X11/xdm/Xsession FluxBox
chmod +x FluxBox
open up kdmrc and find the entry:
And add FluxBox, I put my entry in after XFCE, but it's really your choice.
Now, save and exit. Restart your X server with CTRL ALT BACKSPACE, and you should now have Flux in your KDM/GDM menus, and it should work to execute them.
Also, I am not sure what happen when you tweaked that file, but I will be happy to post my entire config...
Ok, better idea, here's a link to them on my box:
Go in there and I will have copies of all files involved, you can compare and change as needed.
Give me a few minutes they will be in there by 1230 PST, that's roughtly 1/2 hour from now.