@SelfHatenMarine the guy you bought it from is almost right, you only need to setup the wifi and then you can use the duet web control.
On a Mac, I would use the Terminal.app and the built-in screen command for the serial connection.
I would suggest some quality time with the extensive documentation at https://docs.duet3d.com, especially the Getting Started page.
Also, make a backup of the SD card before changing anything.