Java Decompiler (JD) is generally recommended as a good, well, Java Decompiler. JD-Eclipse is the Eclipse plugin for JD.
I had problems on several different machines
Launch Eclipse,
Click on "Help > Install New Software...",
Click on button "Add..." to add an new repository,
Enter "JD-Eclipse Update Site" and select the local site directory,
Check "Java Decompiler Eclipse Plug-in",
Next, next, next... and restart Eclipse.