If your computer cannot boot from usb, first be sure you checked the bios settings and that usb boot is enabled. It's not entirely clear what your purpose is.
If you already have Ubuntu installed, you could use grub to boot the usb -- provided it's a full install on a usb, not the live media. You would just start Ubuntu normally, stick in the usb, then
sudo update-grub in a terminal and it will add the usb drive to the boot menu.
If you are just trying Ubuntu for the first time, the easiest thing by far is just to make the cd and boot from that. But it is possible to install another bootloader to the usb, such as Plob, that can boot machines that don't have bios support for usb booting.
Most computers made in the past 5 years should be able to boot from usb. If your goal is to a distro in your pocket than you should be safe with a standard live media. (And best to make it persistent as well.)
If you think that you may need to use it on unknown computer that can't boot from usb, then definitely look into Plop.