Hello Insoluble25x
There have been issues with the installation of the game through Gamestop's download client. If you have activated the game before you can now reinstall the game through the Uplay PC Client. Below are directions on how this is done.
1) To find the download for the Uplay PC Client you will need to go to
http://uplay.com/
2) Once at the site you will see the orange download option in the upper right corner. It will say Download Uplay PC.
3) Once the Client is downloaded and installed it will ask you to log in with your Uplay account. Now that you are logged into the account that you activated the game on you will see an option to redownload your game.
Chris
Ubisoft Support