You may need to add a JDK (Java Development Kit) to the installed JRE's within Eclipse
Go to Window->Preferences->Java->Installed JRE's
In the Name column if you do not have a JDK as your default, then you will need to add it.
Click the "Add" Button and locate the JDK on your machine.
You may find it in this location: C:\Program Files\Java\jdk1.x.y
Where x and y are numbers.
If there are no JDK's installed on your machine then download and install the Java SE (Standard Edition) from the Oracle website.
Then do the steps above again. Be sure that it is set as the default JRE to use.
Then go back to the Projects->Generate Javadoc... dialog
Now it should work.
Good Luck.