Double click on it to start the Server application.
Note: If you've downloaded the package to "Downloads" and then try to run it from that location, on OSX "High Sierra" it may happen (if for the folder you don’t have permissions to run an app) that the app will not start. Please move it to the Desktop and run from there.
Note: If you get a warning message about an unidentified developer, then instead of double click, right mouse click the icon and choose: Open.
Choose: OPEN in the following window.
Start with running the server in demo mode.