Many people has had this problem with the latest build. (including myself, but got it to work.)
I suggest you first try to browse to folder; C:\Program Files\Lavasoft\Ad-aware SE
See if there is a file named defs.ref
Rename it as defs.ref.old
and see if you can update now. Also make sure you're firewall is allowing access to the update server. If this doesn't work, try and delete all the def files from the Ad-aware folder and now try to update. If none of this works,
you'll need to update manually here;http://www.lavasoft.com/
Except - I'm not sure if the site has been updated yet, so it may be you won't get the latest defs for a while!
There is also many Ad-aware SE help forums available.. Unofficial of course, but it contains Ad-aware Experts.
What also would help to know, if you have an purchased version of Ad-aware or the free version in use -- since there is free email- support for registered users of Ad-aware.
Hope this helps.