Downloading the Game Problem.

Help please! Im having a problem regards to downloading the game. Everytime i open the file. This "Host_SingleInstance" pop up. Can anyone help me please or atleast give me a step by step on how to install the game or is there any download client? I really want to play this game. 

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http://www.playrequi.../clientdownload

->

http://www.playrequi.../clientdownload

i reinstalled a couple up weeks ago using this, so I can verify that this works (for me at least)