There is an error with eclipse 1.1 that doesn't sometimes prevents installation from their online repository. But this doesn't seem to be the case with you since it it says no repository found. Maybe the beta versions have a different URL?
I would suggest downloading it to a local disk and installing it from there. The exact procedure is -
What i did was i downloaded BlackBerry JDE Plug-in for Eclipse for JDE 4.7 and 4.5 seperatly and from which you get zip files. Then go to software updates and then to available software. Click on add site button and select archive button. Browse whre have downloaded the Plugin archive and install it. After you have installed all the JDE plugins you will get options for changing the JDE version.
Edit: I just came across this thread, and the method described above might possibly not work - http://stackoverflow.com/questions/1903926/how-can-i-install-the-blackberry-v5-0-0-component-pack-into-eclipse