Where does your Signalink connect to on the 891? Doesn't it connect to the rtty/data jack on the back? Now, if you mean the USB port, did you install the drivers for the USB port? It won't appear until you've installed the SILabs drivers for macOS, available https://www.silabs.com/documents/public/software/Mac_OSX_VCP_Driver.zip. Make sure the USB cable that connects your FT891 to your computer plugged in, or else the install will fail. Once installed, you'll have to authorize the driver to load by going to the Security & Privacy Pane of System Preferences. Once authorized, then plug in your USB cable and you should be able to get it to work just fine.