Note: You need to do this in the Navigator View.
The Package Explorer View does not show *.class files.
To open the Navigator View, go to
Window > Show View > Navigator
Window > Show View > Other... > General > Navigator
There are many situations in which Eclipse opens a *.class file: CTRL-Clicking into a class/method/field without source code, Stepping Into in Debugger, etc. In such situations, Eclipse is using the default editor for the *.class file type.
Bytecode Visualizer is set as default editor for *.class files during the installation.
If it does not suit your needs, you probably want to adjust the File Associations in Preferences:
Window > Preferences > General > Editors > File Associations
Select the file type *.class and set the editor of your choice as Default
The behavior of Bytecode Visualizer is adjustable in many respects, such as:
Just go to
Window > Preferences > Dr. Garbage > Bytecode Visualizer
and change the settings to suit your needs.