Fedora usually detects windows but doesn't usually detect other Linux distros. See posts about this all the time. Another way is to use this entry in your Fedora /boot/grub/grub.conf file:
You may need the savedefault and boot lines. If it doesn't work with the above entry, try removing the hash marks (#) at the beginning of those lines. You will also need to change the root line to reflect the partition Ubuntu is actually on. Fedora is grub legacy so partitions begin counting at zero. You will need the core.img file but if you were previously booting Ubuntu, it will be there.