Adorable -- if you're looking under your Account Details, it will show you that you purchased the game, but doesn't provide a link for installing it.
If you go to the website by typing in the web address and then log in, I don't THINK there's any way to get to the "Games" menu with the installation program.
You have to actually click on the Steam icon on your desktop, log in that way, and then look for "Games" in the top left hand corner.
The system was clearly designed by a Geek for other Geeks.