It’s a little difficult to find the rescue mode in Ubuntu 11.10. Since 11.04 you can press the shift key in startup phase to show up the grub boot menu.
First step is changing the following line in /etc/default/grub
Activate the changes by running /usr/sbin/upgrade-grub.
Now you can boot up your system and can press the shift key to access the grub menu.
Another important thing is that the file systems are mounted read-only in rescue mode. If you want make changes to the file systems then you must first choose the remount option in the boot menu.