High Sierra and Mojave Operating Systems have a new security preference in place that will block the installation of drivers.
After initiating the SPD-SX driver install, go into the System Preferences on your mac and click "Security and Privacy."
Click "General," and first confirm that there is no message showing that the software is being blocked.
Click the "pad lock" symbol in the lower left corner and then type in your password to unlock it.
Click the round button to allow for: "App Store and identified developers" in the "Allow apps downloaded from" section.
Even if it is already selected, click to highlight "App Store" and then click back to App Store and Developers to confirm.
After initiating the driver install process again, the SPD-SX will appear in this same menu and you will need to authorize the driver installation -which may include inputting in your mac password.
(Note, it may take several attempts to get the SPD-SX drivers to appear as blocked).
After you have approved it, the driver installation will continue and complete - prompting you to restart the computer.
Lastly, be sure you're connecting to the USB COMPUTER port on the back of the SPD-SX (not the USB MEMORY port), and you're connecting the SPD-SX directly to a USB port on the mac. (USB hubs can be problematic and cause connection issues).
Here is a link to further USB connection information: