That add-on is old. You don't need one anymore. It works natively because they changed the way Boxee requests video from Hulu so that it opens a webbrowser and goes to Hulu that way (you can briefly see the browser window popup sometimes when starting a video). That way, Hulu doesn't know that you're using Boxee, all it sees is the browser.
I'm guessing at some point Hulu may find a new way to block Boxee, but as of now you can watch videos directly from Boxee's interface.