Well, if you want to log in using your login manager then locate your sessions directory. Mine is /etc/X11/sessions on arch linux. There will be several (depending on whats installed) *.desktop files . you will need to create one for fluxbox and add it here. here is what mine looks like for fluxbox.
Comment=lightweight window manager
# no icon yet, only the top three are currently used
just edit the paths to match yours, log out, then you should see fluxbox in the list.