Hi Amar!
You should be able to use Fedora's grub just fine. I don't think you can use the chainloader option unless you have Ubuntu's grub installed to the boot partition of the Ubuntu install (You say it was installed to the MBR before the Fedora installation)
You will need to find out which partitions contain your Ubuntu filesystems, and create a custom menuentry similar to the ones you see for Fedora.
Here's a link that may help: http://www.fedoraforum.org/forum/showthread.php?t=263739
Once you can boot into Ubuntu, you might want to re-install Ubuntu's grub to the MBR. I believe it will detect the Fedora installation automagically, where your Fedora installation is failing to detect Ubuntu.
I hope this helps!