I downloaded DJI Assistant 2 to my iMac successfully according to the installation package however I can't locate it for use on my computer.

It should be in your Downloads folder. As DJI don't sign their software you'll have to right click the installer and select "Open". It will tell you its not signed and you can install anyway. Not sure why they bundle HoRNDIS with it, but I just selected not to install that (click the red cross when it pops up)