No refunds will be given where the key has been disclosed to you.
Please note: Your order may require a Paypal authorisation check before sending your key to prevent others using your account to make an unauthorised purchase.
For those new to Steam.
You just need to install Steam from http://store.steampowered.com/ (if you haven't already)
Then redeem the code via "activate a product .." in the games menu at the top.
Just paste in the code when you get to the redeem box.
You will then see the game in your Steam library and once you have downloaded it you can play.