USB Ubuntu doesn't boot

I have been trying to make a bootable Ubuntu live usb disk and none of the methods I found online worked for me.
I followed the official instruction from here but the drive didn't even show up as an option when booting up.
I also tried the unofficial method described here using a Ubuntu VM and this time I got the "Missing Operating System" error before the installation could begin.
Installing rEFIt, using the latest 11.10 beta and the 64bit version didn't work.
I don't have an optical drive in my 2011 MBP so I NEED to get this to work.

Please help.

 

Thanks but I already tried that and it didn't work.