MacBook cannot find a port to upload sketches

Hello,
my MacBook with OSX High Sierra works fine with the IDE and the WebEditor but it does not find a port for uploading sketches.
I downloaded and installed the plugin and after that the plugin-icon shows up on top of the screen. During the installation a message showedup saying the installation process was ended. I don't know with or without success. With that icon I can start the WebEditor but I cannot upload a sketch. Any idea?
Thanks!
Ernest