This has been tested on Xp, 7, wheezy and ubuntu. (using DD should just work on all linux distrbutions..)
There is an issue writing hybrid images to a usb using unetbootin/ubuntu's startup disk creator as it seems to not write them correctly.
When you run it - you will get a screen like this.
Select your ISO - you have to change the file type to *.* (all) to see ISO's as it by default only displays IMG files
Select the USB device to write to. Be careful - it will erase the entire usb stick..
Then click Write - you will get this warning for your selected usb device.
Then it will write the image to the usb stick.
At the end you get this message.
At this point - you can reboot your computer and select to boot from the usb. (every computer is different - check with your motherboard/system manufacturer..)
Using DD to write the hybrid iso:
Plug in your USB stick, and open your terminal window.
Use dmesg to determine which device is your USB stick:
% dmesg [...] [2410426.237358] scsi 24:0:0:0: Direct-Access Verbatim STORE N GO 5.00 PQ: 0 ANSI: 0 CCS [2410426.238045] sd 24:0:0:0: Attached scsi generic sg4 type 0 [2410426.903202] sd 24:0:0:0: [sde] 7823360 512-byte logical blocks: (4.00 GB/3.73 GB) [2410426.904123] sd 24:0:0:0: [sde] Write Protect is off [2410426.928097] sde: sde1
Your output will be slightly different, but pay attention to the bolded parts above, and make sure you have the right device. You can see mine is a 4 GB "Verbatim" stick, and it is given the device name sde. It is important you know the right device. Do not guess.
An alternate command to find the correct device is lsblk which will show all devices and their mount points by default. Run it once with the USB device unplugged, then connect it, give it a second to find it, and run lsblk again and the new entry will be your device. Again, be careful, DD is a orbital nuclear laser of power.
Now use dd to write the image:
% sudo dd if=XXXXX.iso of=/dev/sdX conv=fsync
Of course, you'll replace XXXXX.iso with the name of your downloaded iso, and sdX with your USB stick's device name.
When it is done writing, this USB stick will be bootable.