You can do this through right-clicking a *.java file in e.g. Package Explorer and the going to
Open With > Sourcecode Visualizer
After you have opened the Sourcecode 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.