Viewing the decompiled source for a class is as simple as opening any class in a jar which doesn't have a source attachment.
Switching between decompiled/regular source for all classes in a jar can be done quickly by clicking the buttons in the toolbar.
These buttons are only visible when a class editor is active or a class, jar package or entire jar is selected in the Package Explorer or Project Explorer.
You can also right click on a class within the Package Explorer or Project Explorer view to control decompilation. However, if you choose to open any class in a jar which doesn't have a source attachment, decompiled source will be automatically attached to the jar.
The JD Java Decompiler outputs line numbers in comments. These are used by JD-Eclipse Realign to line up the decompiled source correctly. Once this process is complete the line number comments can be optionally stripped out.
Further details about how this plugin works can be found on my blog:
This project adds the following: