I am attempting to get my S90 ES communicating via USB on my iMac using Mac OS Mojave and having no success. I installed the latest Yamaha/Steinberg Midi Driver, and my S90 ES is set to communicate via USB. I created an external device in my Audio/MIDI Setup app, but so far have not been able to get the keyboard to communicate with Garageband or Logic.

You installed the wrong driver. The S90 ES does not use the “Yamaha/Steinberg MIDI driver” (sic) (there is no such Driver). The driver must be specific. Any driver that starts “Yamaha Steinberg” is for a product that does both audio and MIDI via USB — This does not include the S90 ES.

You want to get specifically the “Yamaha USB-MIDI Driver” for your Computer Type and Operating System.
“Yamaha USB-MIDI Version 1.3.2-2 for Mac OS”. This is a MIDI Driver (only). The S90ES does NOT do audio via USB.

S90 ES Download Page

USB MIDI connection between an S90 ES and Mac running 10.14.6 / Mojave works great here. Once connections are made properly, you get the attached icon in your Audio MIDI Configuration.
🙂

...too bad that Yamaha's S90 ES voice/multi editors are too old to run on any current Macs.