After you have installed the plugin create a project or open an existing project.
To create a new XD project go to File > New.
The New Project screen appears. Choose any size template. You can change the size later.
If you don’t have a project to work with you can download the Wires - Web.xd or Wires - Mobile.xd project.
You can find these projects from within XD by going to File > Get UI Kits > Wireframes.
The URL to the Wires projects at the time of this writing is here.