I understand you're looking for the MIDI driver for the Yamaha PSR-S550 keyboard.

Part 1: Do You Actually Need the MIDI Driver?

Before diving into downloads, let's clarify a critical point. The Yamaha PSR S550 is a USB-MIDI class-compliant device.

The Yamaha PSR-S550 uses the standard Yamaha USB-MIDI Driver

For Mac Users: Apple dropped support for 32-bit drivers and kernel extensions. The official Yamaha driver for PSR S550 does not work on macOS Catalina (10.15) or newer. Mac users must use the class-compliant mode (plug and play) or use a MIDI interface (5-pin DIN).