First, close everything Battle.net related, even the agent from processes list in task manager.
Second download this:
MEGA
Create a new folder that you want to install alpha in and extract the rar into it.
Load up Battle.net and scan for games in the application. Choose folder with this files.
Go to the directory of your Battle.net appplication (Where the battle.net launcher.exe is located)
Shift right click somewhere in the directory (In a empty space)
Open command window here.
Type "battle.net.exe --game=wow_beta --repair" without the quotes.
When repair process will be done type "battle.net.exe --game=wow_beta --install" without the quotes.