With just a few clicks you can enter the code via your internet browser:
1. Open https://store.steampowered.com/account/redeemwalletcode
2. Sign in with your Steam account name and password.
3. If you don't have a Steam account yet, create one.
4. Enter the code in the field "Steam Credit Code". Click on Continue". The top-up amount will be added to your Steam Wallet balance.