HelloGrandpamike, Welcome to the forum.
Let me see if I can assist you.
Press the Library tab
At the bottom Left in that window is a link that says" + ADD A GAME... in white letters.
Press that and choose activate a product on steam
Another box will appear asking you To register your product with steam, Blah, Blah, Blah
Press next to continue.
Press I agree.
You will see a window that you can either type the product key, or paste your key there if your copied the key to clipboard Ctr+C
It will do its thing a second or so than should start the download.
When that is complete then go back to the library and on the left choose the red icon GameGuru icon. Then in the right window you will see the word install perhaps. Press that and let it install and finish that.
When that is complete, that install will turn into Play. Pressing that will launch GameGuru for you. Hope that helps.
I will Check back and see how it went for you. Happy creating.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit