Where do I enter a Steam game key?

To activate a Steam game key, open the Steam client, click Games in the top menu, then select "Activate a Product on Steam..." and follow the prompts. You can also activate via browser at store.steampowered.com/account/registerkey. Note: Steam Wallet codes (gift cards) are entered separately — go to Account Details → Add funds to your Steam Wallet → Redeem a Steam Wallet Code.