<extension point="org.eclipse.ui.popupMenus">
     <objectContribution
       objectClass="org.eclipse.jdt.core.resources.IFile"
       id="org.threeriversinstitute.ddjexample">
       <action
         label="Hello"
         class="org.threeriversinstitute.ddjexample.HelloAction"
         id="org.threeriversinstitute.ddjexample.helloaction">
       </action>
     </objectContribution>
</extension>

Example 1: Declaring an extension point in the plug-in manifest.