BIOS modify to boot into usb?

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.

See > Startup Manager: How to select a startup volume

If not, then see > Starting from an external USB storage device (Intel-based Macs)

what if you had starting using bootcamp to install windows and when it booted up it wont read the usb but i cant get it back out of the windows boot up and it keeps asking me for disk when i boot it up is there a way to change the settings to boot the usb from the windows install window?

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.