ThisWorkbook.cls needs amending as well to work on Mac
So, while my changes to the .bas are sufficient to get the procedure to work on both Mac and Windows, I neglected the class file. Just as with other add-ins that are capable of doing so, it should be possible to amend the class file to write a menu item into the Excel gui. For instance, there's something called the Power Utility Pack that's able to do just that with Excel 2011.
I think it would still be a boon to accept the pull once you're confident that it works in both environments. Sometime soon, I plan on altering the class file for the aforementioned stuff.
EDIT: I'm such a noob. The above might be a mistake, plus I just realized that I did not change the actual module embedded in the xlam file -- d'oh. Postpone and I will update you soon.
EDIT: This can completely be ignored and closed. I edited the .xlam and submitted pull request.