•  Class or resource files
  For simple custom actions, custom screens or other handlers that do not depend on other classes, it is easiest to insert their class files directly, especially if you build your installer extensions together with your application. Anonymous inner classes will be included automatically. If you select a resource file, e.g. an image, it will be added to the top-level directory of the custom JAR file and will be available via Class.getResourceAsStream().
•  Directories
  With this type of entry you can add an entire directory. Please make sure to select a classpath root directory, otherwise your classes cannot be loaded.
•  Archives
  With this type of entry you can add a JAR file.

Use the [...] chooser button to select files and directories from your file system. A relative path will be interpreted relative to the project file.