Had the same "issue" trying to customise boot of Eee PC 1005PEB. A quick search of the forums using keyword "menu.lst" revealed the answer.
BT5 appears to use grub2 not grub, the config file is now /boot/grub/grub.cfg. a quick "nano /boot/grub/grub.cfg" shows that you should not edit the file directly as it is generated by "/usr/sbin/grub-mkconfig".
For yours and others convenience maybe checkout this forum post:
*then read up on Grub2