Again, I know it's an old post, but for the Googler's out there... here was my solution.
Eclipse 3.2.2 and I had only JRE 5.0 (how Eclipse was running without javac or JAVA_HOME no one really knows...)
Download the JDK (development kit, not runtime environment) and point Eclipse to that. This, by proxy, will get "javadoc" added, and it will magically work!
JDK5 Download After you have downloaded, install - such as C:/Program Files/Java/JDK5...
Java builder: Windows -> Preferences -> Java -> Installed JREs -> Add the JDK installation.
This solution worked for me without restart, although Eclipse did rebuild all of my open projects.