Boot to single user mode (add '1' to the boot parameters), like:
boot hd:6 boot/kernel root=/dev/sda7 1
once at the # prompt, mount / as read-write:
mount -o remount,rw /
edit /etc/fstab in your favorite text editor, and add a new line to the end of the file:
Save it, and reboot.
If you can't get to single user, please use the 'rescue' option of the installer, mount your / partition, and edit /etc/fstab. Note, that some editors, like my favorite 'joe' automagically break long lines, which results in broken /etc/fstab.