You can access perferences in othe plugins via the PreferenceService. For this example create a new Eclipse plugin "de.vogella.preferences.access" (not RCP) and add the following command handler to the menu.

			
package de.vogella.preferences.access.handlers;

import org.eclipse.core.commands.AbstractHandler;
import org.eclipse.core.commands.ExecutionEvent;
import org.eclipse.core.commands.ExecutionException;
import org.eclipse.core.runtime.Platform;
import org.eclipse.jface.dialogs.MessageDialog;
import org.eclipse.ui.IWorkbenchWindow;
import org.eclipse.ui.handlers.HandlerUtil;

public class AccessPreferences extends AbstractHandler {

	/**
	 * the command has been executed, so extract extract the needed information
	 * from the application context.
	 */
	public Object execute(ExecutionEvent event) throws ExecutionException {
		IWorkbenchWindow window = HandlerUtil
				.getActiveWorkbenchWindowChecked(event);
		String text = Platform.getPreferencesService().getString(
				"de.vogella.preferences.page", "MySTRING1", "hello", null);
		MessageDialog.openInformation(window.getShell(),
				"Access for preferences in different plugin", text);
		return null;
	}
}

		

Add this plugin to the launch configuration of plugin "de.vogella.preferences.page" and you should be able to access the preferences of "de.vogella.preferences.page".