Now with board manager, the board packages are stored elsewhere. I don't know where it is on mac, only windows.
If you just need support for 20mhz ATMega 328p, might I recommend this core? It supports 20mhz out of the box, can be installed and updated through board manger, works with all the atmega x8 series (ie, 8, 48, 88, 168 and 328 p and non-p) and is being actively and competently maintained: GitHub - MCUdude/MiniCore: Arduino hardware package for ATmega8, ATmega48, ATmega88, ATmega168, ATmega328 and ATmega328PB