I selected the primary display for the notebook and the secondary display for the external monitor.
How can I set up the bottom panel to be displayed on all monitors?
You can add a panel on to the additional monitor by right clicking on panel, add panel. You then need to click on the properties for the newly added panel and uncheck expand. You should be able to drag the new panel to the additional monitor then check the expand box and add any apps to the new panel. Unfortunately I am unaware of a way to extend the existing panel or easily duplicate the original panel.
Hope that helps.