Release date: 20/10/2011

PAYDAY™ The Heist is an action filled first person shooter that lets players take on the role of a hardened career criminal executing intense, dynamic heists in constant pursuit of the next “big score”. Load out with an array of weaponry and equipment. Navigate six high-stake heists with three other live or A.I. Co-Op partners in crime.

Time for a PAYDAY™ - Cash in Before you Cash Out!
Features

Six High-Intensity Heists

Interactive 4-Player Co-Op play

Endless Replayability

Massive Character Progression Tree

Build Your Skills to Fit Your Preferred Form of Violence

Hostage Trading

More DLC: Additional heists, weapons and equipment means PAYDAY™—a digitally distributed AAA quality title for less than half the price of a retail game—is total OVERKILL

Go to: http://store.steampowered.com/ and download STEAM client
Click "Install Steam" (from the upper right corner)
Install and start application, login with your Account name and Password (create one if you don't have).
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Launch Steam and log into your Steam account.
Click the Games Menu.
Choose Activate a Product on Steam...
Follow the onscreen instructions to complete the process.
After successful code verification go to the "LIBRARY" tab and start downloading.