Using UNETBOOTIN is very easy. Just install it by typing:
sudo apt-get install unetbootin
Then run it by pressing Alt+F2 and entering
Enter your password and then you'll see the main menu. Tick the second radio button (Disk image) and browse to the ISO file of the distribution you want to make the live disk of. Make sure the pendrive is empty, or else format it to FAT through Disk Utility. Mount it again if you format it, and it'll be detected by UNETBOOTIN. Then the wizard will do the work and you'll have you disk ready.
But remember, there is difference. Unetbootin creates a live version of the distro, which is primarily used to test it in live mode (without installing) or for installation of the distro to a system. On the other hand, Startup Disk creator makes your usb stick ready as a distro itself, so that you can use it as a portable OS.
Since you say that your system is showing boot error, I suggest you reset your BIOS and set it to boot again from USB drives. Besides, you should format your drive once, before creating the startup disk. Hope it helps.