Trying to install on Mac (failing)

Trying to install on:

iMac (Retina 5K, 27-inch, Late 2015)
Processor: 3.3 GHz Quad-Core Intel Core i5
Memory: 24 GB 1600 MHz DDR3
Graphics: AMD Radeon R9 M395 2 GB

PortableMacAppsHelper.app is located on an external SSD drive connected via USB. Confirmed to be writeable. Downloaded today.

Tried on two separate machines (OSX) and get the same error each time:

"Error! Can't initialize plug-ins directory. Please try again later".

This looks to be a WINE error, but Googling shows this usually happens in Windows.

Any pointers to what I'm doing wrong here?

TIA