To use C18 with MPLAB 8.92, install C18 before or after MPLAB, then set the compiler path in Project → Set Language Tool Locations .
https://ww1.microchip.com/downloads/en/DeviceDoc/MPLAB_IDE_8_92.zip microchip mplab 8.92 download
MPLAB X is designed to work seamlessly with Microchip’s hardware debuggers (PICkit, ICD, MPLAB Snap, and Real-Time ICE). To use C18 with MPLAB 8
: It is a mature, "frozen" environment with no new bugs being introduced. To use C18 with MPLAB 8.92