I don't use a mac, but I can tell you how I do my cutout renderings for my products in the windows version.
Everything happens in the Render tab.
1- you select Maxwell environment obviously
2- you click on Transparent background
3- In the Maxwell for Rhino tab you go to OutPut
4- You select an image format that accepts transparency such as PNG for example
5- In Channels, under Output Mode you choose embeded rather than separate to integrate the alpha channel into your file.
6- checkmark render All
7-check Alpha auto
8- Then I check Opaque, but I don't know if it's useful
9-In general, I add Material Id and Object ID as soon as I have modifications to make.
On Windows, there is a bug:
Sometimes I get images on a black background and not on a transparent background; so I use my layers to regain transparency.
But the best is to re-save the image from the MXI in the render, by choosing to save in png for example, by saving all channels by checking the option embed channels if the format supports it