(For posterity,) the "Update Required" is because High Sierra is (probably) using the Apple NVIDIA driver. You need to install 1. NVIDIA's web drivers, 2. their CUDA driver and 3. the correct CUDA libraries for your "updates version" of Mac OS.
- NVIDIA's web drivers
- their CUDA driver
- the correct CUDA libraries for your "updates version" of Mac OS.
Finally as always with NVIDIA drivers, the version you use has to be compatible with the version of the software you are trying to pair with: for example - Xcode 10.1 will work ok with CUDA 10 drivers but not CUDA 8 drivers. The same is probably true of Mathematica.