kdm should automatically find that you have installed fluxbox and will display it as a choice.
fluxbox startup script
By this way it is not possible (at least in my Debian 3.1 Sarge with kdm) to use ~/.fluxbox/startup script, but in Debian 4.0 Etch it seems to work.
For Debian 3.1 Sarge, first edit /usr/share/xsessions/fluxbox.desktop:
[Desktop Entry] Encoding=UTF-8 Name=Fluxbox Comment=Highly configurable and low resource X11 Window manager Exec=/usr/bin/startfluxbox Terminal=False TryExec=/usr/bin/startfluxbox Type=Application [Window Manager] SessionManaged=true
changing the bold lines: it's started /usr/bin/startfluxbox script, not just fluxbox. Finally edit ~/.fluxbox/startup:
#!/bin/bash ## uncomment it to launch xscreensaver or idesk ##xscreensaver & ##idesk & exec /usr/bin/fluxbox
Now it is possible to start a fluxbox session from kdm and use the startup script ~/.fluxbox/startup to fit your needs.