If you already have Eclipse installed, you are only required to add Papyrus to your configuration.
To create a new Project, navigate to File -> New -> Papyrus Project. Enter your project name and, if desired, change the default location. Advance to the next screen, check the UML box, and press Finish.
A file called "model.di" will now be open and ready for editing under the Model Explorer. You may find elements relevant to you within the Palette window. Use Cases, Actors, and Subject Area are all contained within the Nodes category and the arrows used to relate these elements are contained within the Links category.
You may add objects by selecting them on the Palette and clicking anywhere on the model.di whitespace. To edit an object's attributes, select the object and change the desired fields on the Properties window. To create a relationship, select an arrow, click the first actor, and release the mouse over the second actor.
Note that the file you need to download in SourceForge, is located within a folder called Image Export Plug In.
To export an image, navigate to File -> Export.. -> Other -> Image of Current Diagram, select the desired Image Format, and press Finish.