If you're planning on exploring the Atlas Control Toolkit, don't download the July CTP.  In that release, there are problems with the toolkit's designer support for control extenders.  To see one discussion of the problem, check out this thread on the ASP.NET forums.  The fix to this problem was released in change set 2691 on CodePlex.  Simply download any version after that, and you should have no problem.  The fix will, of course, be included in the next CTP as well.