Alienware M11xR2 doesn't offer boot from USB in BIOS

I'm trying to install a Ubuntu partition and used Unetbootin to make a USB which should act as an install CD. It works fine on a friend's laptop (Dell latitude d610 or something) but it doesn't work on my Alienware M11X.

I went into the BIOS to configure boot options, and the only devices it's recognizing that can be booted from are the hard disk and the network card. The manual says that it should offer the option of booting from USB, and gives no advice on how to obtain that option when it isn't showing up by default. I updated the BIOS to A03, and that didn't solve the problem. I've tried all 3 USB ports with BIOS A02 as well as A03, and none of it has worked.

Has anyone else had similar problems with the M11X refusing to see a USB to boot from? Any advice on how to solve this problem? I chose Alienware over other brands partly because of Dell's excellent service and support, but I've been extremely frustrated in seeking help on this issue.

Thanks!