You can sure use UNetbootin. Although there are a few important things to be aware of, or else it won't work:

First, be sure to format you usb key to NTFS and flag it as bootable -- you can use gparted to do that (sudo apt-get install gparted). Then run UNetbootin with your usb key already mounted. On its interface, select "Diskimage" and then browse for your .ISO file. Check the "Show all drives" box (so it can find your NTFS formatted usb key) and select the right drive (be careful!) -- on my case, it was "/dev/sdd1".

Click "Ok" and wait. It takes a long time to finish copying all the files. So wait.