Not sure about actually installing the grub rpm from a rescue disc, I don't know anything about FC5. But it's easy to set it up manually. Once you get booted, you can install the rpm properly.
You want all the stage* files from the rescue disc, put them in /boot/grub on the hard drive. Then run grub and do a
where X is your drive number (starting from 0), and Y is the partition number, again from 0, e.g. /dev/hda1 is (hd0,0), /dev/hda2 is (hd0,1), /dev/hdb1 is (hd1,0) and so on.
It will tell you if it's successful. Assuming it is, create /boot/grub/menu.lst with something like
kernel /boot/grub/vmlinuz ro root=/dev/hdaX
and give it a try. If it doesn't work, hit e at the grub menu and fiddle with the parameters. Once you get it working, you can reinstall grub from rpm and edit the menu.lst with the working parameters. Note that if you move any of the stage files, you need to run grub and 'setup (hdX)' again. After you get it working, making the stage files immutable is a good idea (chattr +i /boot/grub/stage*), to prevent accidents.
Note that the /boot/grub/vmlinuz path assumes that /boot is not on a separate partition. If it is, you just need /grub/vmlinuz, since it is relative to _grub's_ root device, not the system root, which may be different (i.e. the grub root is whatever partition /boot is on).