The plugin is deprecated like the Jenkow plugin
The Activiti Designer is an Eclipse-based workflow editor. It is part of the Activiti BPM Platform project. (The Activiti Modeler is another, web-based workflow editor.)
The Jenkow Activiti Designer plugin encapsulates the Activiti Designer together with the Designer extensions specific for Jenkow and offers them as a single Eclipse update site in Jenkins (using the Eclipse Update Site Plugin).
The update site is used as a source repository for installing the Activiti Designer in Eclipse.