In order to activate this code and fill up your balance, you need to:

Copy the code that you’ve received.
Visit http://daddyskins.com, sign in with your Steam account (or alternate method)
Press ‘Filling up your balance’ button on the top right which stands for filling up the balance
In the opened dialog, choose "Gift Cards" tab, paste the code you've received into the "Gift card code" field and press the button next to it to see the dollar amount you are about to add.
Press "Add Funds" for filling up the sum equal to the price of the Gift card you've purchased.