The Eclipse Plugin is developed and maintained by Hallvard Trætteberg (many thanks for his work!). Like the core library PlantUML itself, it is open source and the plugin is distributed under EPL license. The source code is hosted on GitHub.
First, you have to display the PlantUML View (click the Window menu): This view displays automatically the class you are working on: If you write some comment in PlantUML language, the corresponding diagram is automatically displayed. And if you have several comments with diagrams, it selects the one the cursor is in. In the Preferences Windows, you can also set up GraphViz path if needed:
The plugin is not limited to Java source file, it also works with Ecore/Xcore files. So that you can see the corresponding class diagram in a view side-by-side the Ecore/Xcore editor : If you want to support other file types, you can implement a new extension to do so. You can have a look at the current xcore implementation.