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
I made the steps 1, 2, 3 and the 7. and I put the folder with the class files in the project build path (right click, properties, java build path, libraries, add class folder, create new folder, advanced>>, link to folder in the file system, browse,...) then restart eclipse.
Simple thing i did to get it working:
Went in eclipse > Window > Preferences
(Optional)typed in the search box "file" to help trim the tree of options. Went to General > Editors > File associations.
Clicked the ".class" type. Below there were 2 editors present, i clicked on the "Class File Editor" - the one with the icon from JD, clicked the "Default" button on the right.
Done. Now all ur class are belong to us.