In my case I needed to install "JST Server Adapters". I am running Eclipse 3.6 Helios RCP Edition.
Here are the steps I followed:
Help -> Install New Software
Choose "Helios - http://download.eclipse.org/releases/helios" site
Expand "Web, XML, and Java EE Development"
Check JST Server Adapters (version 3.2.2)
After that I could define new Server Runtime Environments.