Many users assume they need a PC to redeem a Steam game code, but you can easily do it through the Steam Mobile App. The process is slightly hidden within the app menu, so here is exactly how to find it.
Tap the icon (often found in the "Store" menu or by tapping your profile/account balance). Select "Redeem a Steam Gift Card or Wallet Code" . Enter your game key into the text box and tap Continue . steam mobile activate key