Select the Plug-in editor's Dependencies tab:

 

The Dependencies tab page shows the dependencies that your plug-in has on other plug-ins.  You must list on this page all the plug-ins that contribute code required on your plug-in project's class path to compile.

Now click on the Add... button in the Required Plug-ins section to bring up the Plug-in Selection dialog:

 

 

Select a Plug-in and click on OK. The Plug-in will be added to the list of Required Plug-ins: