Configuring JD-Eclipse Java Decompiler plugin for Eclipse Java EE IDE

I installed JD-Eclipse to the exact specifications of the directions here. Now when I navigate to a .class file via File > Open, I receive this error:

The Class File Viewer cannot handle the given input ('org.eclipse.ui.ide.FileStoreEditorInput').

Do I have to configure the plugin, or do I need to add the .class file to a project repository or something like that?