I have a;

Fedora install on my Computer.
I have a linuxmint-14-Kde.iso on DVD.

How do I put Linux Mint on a USB stick ?

Two possibilities suggest themselves:
1. if you want a live-usb with persistence, something like unetbootin or Mint's (or Ubuntu's) live usb creating tool should work.
2. A full install as you would on a hard drive (don't know if ubiquity will let you do this)

As suggested above, there are many ways.

One is to follow Mint's web pages or www.pendrivelinux.com for easy ways. Those installs are pretty safe. They use either the native app or a third party app to convert a live cd/dvd to a usb.

Second is a way to just take a special iso file and use dd command to install it to your usb. BE CAREFUL. dd is a command that has no opps key.

Third is a way common to most modern distros. A usb is used like an internal scsi drive. In fact an IDE drive is treated like a scsi drive as well as a sata. All appear to linux as a real hard drive. That means most installers just ask you where you want to install the OS. Be careful here too. RTFQ comes into play. Read the F question. If you hit OK, OK, OK you will be back here.

use the dvd to do a normal install BUT to the usb

use gparted ( in the fedora repos or use the live cd )
to blank the usb
plug in the usb and pop in the install dvd and reboot

do a normal install BUT to the usb
-- it will run a bit slow

there are a few other ways including using "dd" but that is on a " LIVE " version and in iso format

jefro is right the sure way is with Pendrive.com don´t use Unetbootin i tried it and never had luck with it
so all the Linux distros ISO i made with success was with Pendrive.

I just download the Mint ISO, saved it and:
(as root)
Where /dev/sdc is the path of the unmounted USB stick. Check in /var/log/syslog what path the USB stick got when it was plugged in.

If you use the wrong path here, you destroy the wrong drive.

jlinkels