Hi Martha, I know the frustration :( (And many answers on the Arduino user forums don’t seem to work). For me, the critical thing is to have a driver installed — even though in the guidebook we were told that no drivers should be needed. This YouTube video (it even provides a working download link in the info section) helped me download and install the missing driver. Also, the serial connection between Mac and the Uno board is quite finicky in my experience. Even with the driver installed, I still needed to unplug and replug it back in for the connection to work, and when working through projects, the connection often drops and requires several rounds of this old-fashioned manual reset. Just a heads-up :) Best luck Martha. Hope this helps!