I have not had a chance to properly test this (i.e. run the macro on actual data), but the method to install Andrew F. Hayes‘ PROCESS macro for SPSS (Version 24) on macOS Sierra seems to be:
Extensions / Utilities / Install Custom Dialog (Compatibility mode)
and then open “process.spd”
This then adds a new menu item to:
Analyze / Regression
PROCESS seems to be available after closing and reopening SPSS.