I followed this guide to install backtrack3:

How to install backtrack 3 final from the shell - Remote Exploit Forums

But I don't want to create a new boot loader, since I already have grub in ubuntu. So I added this to menu.lst:

title Backtrack 3 KDE
rootnoverify (hd0,5)
kernel /boot/vmlinuz vga=791 root=/dev/sda6 ro autoexec=xconf;kdm
boot
I always get this error when I try to boot:

VFS: Cannot open root device "sda6" or unknown-block(0,0)
Pleas append a correct "root=" boot option
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
Edit: I fixed it. Instead of /dev/sda6 i put in /dev/hda6 and it worked. When your in the backtrack livecd pay attention to the partition device id's