The node-red dashboard is an add-on module and is not installed by default. To install click on the top-right corner button and click on the Manage palette.
This will bring up the User Settings screen, click on the Install (1) tab, and type in node-red-dashboard (2) in the search bar. This will bring up the node-red-dashboard module and you will be able to click install (3).
Once installed, the following nodes will be displayed on the left side. To use these nodes, just drag and drop them onto the canvas.