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
or alternatively
Window > Show View > Other... > General > Navigator
After you have opened the Bytecode Visualizer for the first time, you are confronted with the licensing panel.
Click Get free trial license if you just want to evaluate the product.
Click Activate purchased license if you have already bought a license from Dr. Garbage.
For more details, please visit License Key Installation Guide.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