Make sure you have .NET Framework version 4.0 or higher to be able to run Wassapp in Windows 8. Download the package and run it as Administrator and it will. However, it's recommended to use the official WhatsApp application through Bluestacks because it's more secure.
Install Bluestacks and then use the built-in search engines to download WhatsApp. You might need your phone to confirm the installation because you will receive an SMS message with a confirmation code that needs to be entered in Bluestacks.