I just bought a digital copy of Grand Theft Auto 4 from Amazon, and I was given a Steam key to use.
The problem I have is that I do not see any button in Steam that allows you to add a game to your Library.
I've looked under the "Games" tag and have tried the methods described in various YouTube videos, but still no luck in solving my problem.
It's as if my browser is missing a row of selections at the upper part of the screen and, as I said earlier, there is no add a game button visible in the lower left of the Library.
Best Answer
From your question it seems like you're using the in-browser version of Steam (http://store.steampowered.com/).
If this is the case, you will be unable to install your game until you download the Steam application, which can be found here: Steam download
After downloading, installing, and then opening this application, click the large text that says "LIBRARY", and then click "+ ADD A GAME" at the bottom left hand corner of the app. From here, the process for installing your game is very simple, just follow the app's instructions.