I have tomcat 5.5 installed, running and verifiable at http://localhost:8080/. The Tomcat menu option appears in the Eclipse menu bar and I can start and stop Tomcat from there. In Eclipse, it does not show as a Server Runtime Environment in Window - Preferences - Server - Runtime Environments, nor does it appear in the list of environments that can be added when I click the "Add" button. All I see is the J2EE Runtime Library.
Edit:
Running on Windows XP.
Eclipse version is 3.5.1
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:
After that I could define new Server Runtime Environments.
EDIT: With Eclipse 3.7 Indigo Classic, Eclipse Kepler and Luna, the steps are the same (with appropriate update site) but you need both JST Server Adapters and JST Server Adapters Extentions to get the Server Runtime Environment options.