I am not aware of any software for the Megger PAT 420 that runs natively on the Mac Operating System.
There is software that runs on IOS (tablets and phones) however I doubt this will deal with the db (SQL Lite Database) output from the Megger. It may be able to import the CSV output from the Megger however the data has characters in it such as the Ohm symbol) that might cause it to struggle.
There are possibly 2 or 3 options
Option 1 and 2
You can use Bootcamp to dual boot your mac, either choosing to load Mac OS or Windows. I believe Bootcamp comes as part of the Mac OS.
Alternatively you can purchase software called Parallels which allows you to run a copy of Windows within the Mac OS as a separate window.
In either case you can then run Megger Powersuite or SimplyPats or other PC software that can deal with the Megger data.
I would personally use the Bootcamp option over Parallels, however this does mean that you will need to choose which Operating System you would like to boot into when you start your MAC.
Parallels runs under the MAC OS, therefore runs Windows in a Virtual Environment, which can suffer from performance issues. This however does mean that you can effectively run both Operating Systems at the same time.
Either way you will need to purchase a copy of Microsoft Windows OS (you may need to check which version is best)
Option 3
A Windows PC with enough processing power to run Pat testing software (Megger Powersuite or SimplyPats.) can be purchased very cheaply. You may even get away with using a “PC on a stick” which can be plugged into an HDMI tv or standard monitor and allow you to download your Megger 420.
There may be software writers in the future that produce a Mac Version, however the software market for PAT testing is small. Writing a fully featured version that runs natively on Mac OS would be time consuming, given the smaller user base I would wonder what the chance of a commercial return.