Screenshot from 2012-09-17 22:18:43.png
I am using the correct update site "http://download.jboss.org/jbosstools/updates/development/juno/" and as you can see "There are no items available" and when I go look into the log file I see the message that the original poster supplied. I have never heard of or used or seen the update site with Alpha1b.core.
Does the update site work for you? maybe it is a caching thing on our side.