On Playstation 3:
1.First of all please make sure you have a PSN account
2.When playing Portal 2, press the SELECT button, a STEAM INTERFACE will open
3.Follow the on screen instruction. Enter your STEAM user name and Password.
4.You will be successfully logged in, and now you can browse through your Achievements, Steam News, and your Friend List
On PC
1.Get the voucher code from your Portal 2 PS3 copy.
2.Then make sure that STEAM is downloaded in your PC.
3.Now navigate to Games menu, and then select Activate Product
4.Enter your voucher code