To install Audulus as an Audio Unit plugin, select "Install Audio Unit..." from the Audulus menu. You will be directed to a web page where an installer package for the AudioUnit can be downloaded. The installer package will install the AudioUnit to either a user-specific (~/Library/Audio/Plug-Ins/Components) or a system-wide (/Library/Audio/Plug-Ins/Components) location.
The Audulus AudioUnit needs to know the location of the purchased Audulus.app. By default it will look in the Applications folder. If Audulus.app is not in the Applications folder, you will see a button to locate it in the plugin window.