You have two options, the easiest is try putting your ISO in YUMI's run directory. Supposedly it can auto-detect it.
Or you can manually install it alongside (I assume) your other BootCD's on the USB. It's actually a pretty simple process, especially with Ubuntu (others require hacking of the initramfs and mounting and other crazy stuff...)! So, you're lucky to get out easy on this one! Haha!
So, here's what you do:
- Download the 11.04 Server ISO from releases.ubuntu.com
- Extract the CD to a subdirectory of your flash drive.
- Find the syslinux menu. It's called either syslinux.cfg or isolinux.cfg. It will be the one loaded in either /syslinux.cfg /boot/syslinux.cfg /boot/syslinux/syslinux.cfg (or /isolinux.cfg, /boot/isolinux.cfg, /boot/isolinux/isolinux.cfg).
Add these lines:
menu title Ubuntu Server 10.04
- In /ubuntudirectory/isolinux.cfg (or /ubuntudirectory/boot/isolinux.cfg) on every
initrd (it can be either) add this kernel option to the end: directory=/ubuntudirectory/casper
That's it! Try running it in Qemu (use -snapshot!) to be sure if you don't feel like rebooting.