How to boot to the recovery mode if it is not listed in Grub?

The instruction Remove existing kernel options from the linux line and add single refers to changing the line

linux /boot/vmlinuz-3.5.0-26-generic root=<uuid> ro ipv6.disable=1 quirt splash $vt_handoff

to

linux /boot/vmlinuz-3.5.0-26-generic root=<uuid> single

which will boot up and show a text-mode terminal with root permissions ("single-user mode") from which fixes can be made.

The Ubuntu recovery mode is entered by setting linux options thusly:

linux /boot/vmlinuz-3.5.0-26-generic root=<uuid> ro recovery nomodeset

This provides a menu which automates some common system fixes, or lets you access a root shell as the single option would.