For all Joomla 3+ templates built using the Zen Grid Framework v4 (any theme after October 2014) please refer to the Zen Grid Framework v4 documentation.
The panel menu in the Zen Grid 2 template is very easy to set up once you have the correct menu structure created for your menu.
Replicating the panel menu is as simple as following and reproducing the settings as shown in the screenshot below.
You will notice that the only thing that differentiates the panel menu display and the superfish display is the module class suffix -panelmenu. Beyond that setting up the two modules are completely the same.
Also please note that the top level parent items (the items you want to trigger the panel opening and closing) needs to be created as a separator type menu item. If the top level menu item is a link then it will behave as a normal link in the menu and not expand.
Please note that you also need to have the JB Library plugin installed and enabled to use this menu.