Hulu stopped working in Active TV on the DSM-520 in October, 2008. However, if you want to do it for educational purposes...
(from avsforum.com - http://www.avsforum.com/avs-vb/showthread.php?p=13777078)
To get the Hulu beta -- simply open notepad on your PC and copy the text below into it...
<application
url="mymedia.html?containerID=Hulu"
name="Hulu TV"
thumbnailImage="http://www.themediamall.com/online/images/hulu.jpg"
description="Watch your favorites. Anytime. Anywhere."
>
</application>
...then name it Hulu.mcl and put in the folder "Program Files\Common Files\TV-Websites"
Then when you are in Active-TV on the 520 (4th option from the main menu), go to My Media and Programs (1 down from the starting point), and then you will see hulu and cbs -- both of which are in beta.