General usage instructions
You must have Jad installed on your machine. Open the IntelliJad config (see paragraph below for a note on this) and specify the path to the executable in the "Jad executable" field. Once this is done, the existing default settings are suitable for immediate transparent decompilation.
In IntelliJ's settings view, there are two IntelliJad icons - one for project settings, and one for IDE settings. By default, the project settings are disabled and therefore fall back to the IDE settings. In this way, you can set up a persistent configuration for IntelliJad but override it when necessary for specific projects.
Help for configuring IntelliJad can be found under Help/IntelliJad/Configuring IntelliJad/IntelliJad options. The help will be developed on an ongoing basis to improve it.