If the USB drive is bootable and has something on it to boot from, them all you need to do is select it in the Startup Disk Preference
or restart holding the Option key and select it from the Startup Manager.
If not, then see > Starting from an external USB storage device (Intel-based Macs)
What startup volumes do you see when you startup holding the Option Key...? Do you see your Macintosh HD or OS X Install Disc...? If so, then select one of those and startup from that instead of Windows.
What is on the USB, Windows or OS X and it it a bootable volume...? If it is not, then it will not show up in the Startup Disk Preferences or Startup Manager.
I do not use Bootcamp or run Windows on my Mac's so don't quote me on this, but I don't think that Windows can boot from an External USB or FireWire Hard Drive like OS X.
im using osx lion and the options were mac hd and recovery hd when i did options so it aloud me to get back into my mac boot when i selected mac hd but the bootable usb is a fat with windows 7 on it but from what i was reading is that it shouldnt work with a usb because there is no way to change the bios bootup on the windows side so i might have to just burn a copy instead on a dvd.