When you download a steam game on your computer you try to play it but it says only to buy it what should you do

Ok you probably pre downloaded the game which i have done before for my games. You have to pay for the game like lets say i want counter strike source for 19.99 i can pre download the game from the steam network so its like a virtual disk information gets pulled from their network into your computer but for you to install that virtual disk you have to purchase the game once you have done that it will install the game for you just like you just put the disk into the trey.