Hi,
What you have to do is install this plugin from the CB component, not the joomla component. In joomla 1.5 go to Components -> Community Builder -> Plugins Manager
Once you get to that window you will see a list of installed plugins for CB. Scroll down to the bottom of the page an you will see an install new plugin window, similar to the one in the joomla admin. Browse to the file "1.0.1_plug_cbautowelcome.zip" and install.
This is the process I used and everything worked out fine. I hope this helps