I faced the same issue and solved using the below steps. Go to Windows->preferences->Editors->File Associations
Here click on Add then type .class click on OK
again click on Add then type .classwithoughtsource click on OK
Now you will be able to see JadClipse option under Java section in Windows->Preferences
Please provide the path of jad.exe file as shown below.
Path for Decompiler-C:\Users\ahr\Documents\eclipse-jee-galileo-SR2-win32\jad.exe Directory for temporary Files-C:\Users\ahr.net.sf.jadclipse
click on Apply
Now you should be able to see the classfiles in proper format.