Search for people who have NOT registered for an event
First, create a group of people who have registered.  Next, go to Search menu > Custom Searches, and select Include/Exclude by Group/Tag. Include the group of people who should have registered, exclude those who have already registered, and there's your list. 

I believe you can also do a similar include/exclude search using Search Builder in Civi 4.6, but this was broken in earlier versions so I haven't tried.