I downloaded the tarball and it installed ok. Now I don't know what to do... Basically I want to add fluxbox in the Sessions menu in the boot screen. I guess the boot screen must be gdm. any help ???

check out this page :

http://www.linuxforums.org/forum/topic-28225-12.html

Thanks a lot for the link. It installed ok and now fluxbox is placed in the Session Menu.

I have another problem now. when i try to put a background image (jpg or png) in fluxbox i get the following message :

fbsetbg: Something went wrong while setting the wallpaper.
Run 'display -geometry 1280x1024+0+0 -window root /wallpaper/wallpaper.jpg' from an xterm to find out what.

when i run that i get :

*** glibc detected *** double free or corruption (!prev): 0x083cfab8 ***
Aborted

I used up2date and updated everything that had glib in it but the problem remains.

any ideas???

I'm not 100% sure what that error means, I got something similar when I borked a Gtk compile ages ago but at any rate you should download and install feh. It's a great wallpaper setter (<- clearly a technical term) and fbsetbg will recognise and use it straight up.