Configuring eclipse to use jad

Go to Window > Preferences... > General > Editors > File Associations and select .class without source and remove Class File Viewer if it is already there by default and keep only JadClipse Class File Viewer.

Did you change the default editor for class files as stated in the JadClipse wiki?

The Eclipse Class File Viewer instead of the JadClipse Class File Viewer is opened.
Go to Window > Preferences... > General > Editors > File Associations and make sure that the JadClipse Class File Viewer has the default file association for *.class files.

For more recent releases of Eclipse, also make sure that '.class without source' is set to use JadClipse Class File Viewer by default. Installing the plugin will only set the '.class' only by default.