If you want to get started quickly, you can just edit that file and re-launch your server. A better approach would be to add a new OSGi bundle to your server, that provides your servlet's functionality. Your new bundle can contain a plugin.xml file similar to Orion's, that defines your servlet path mappings.
Where is the ZideServlet class defined? If it is in a new bundle, you must add an entry for your new bundle to the server feature (features/org.eclipse.orion.server-feature/feature.xml) to ensure that the bundle is activated at runtime.
Because you are trying to reference ZideServlet from the plugin.xml file of the org.eclipse.orion.server.configurator bundle, you must also add a dependency from org.eclipse.orion.server.configurator to your new bundle. This can be done by adding an entry to the "Require-Bundle" section of bundles/org.eclipse.orion.server.configurator/META-INF/MANIFEST.MF.