The Implementors plugin needs to be made aware of EJB deployment descriptors in order to be able to jump to implementation/interface.
In order to do this, right click on a Java project and show the properties. Here, you can enable or disable the EJB awareness of the project. When EJB awareness is enabled, it's possible to add deployment descriptors (ejb-jar.xml) which refer to interfaces (remote, remote home, local, and local home) as well as to the implementing class. You may add multiple deployment descriptors.