Hi RichTBennett,
Based on the operating system requirement for Office 365 ProPlus, Mac OS X 10.6 or later (Mac) are supported, so the user with Office 365 ProPlus license can install the Office product from Office 365. However, there is no Office 2013 for Mac currently, so the user will install Office for Mac 2011 when they click install in the download page.
System requirements for new Office: http://office.microsoft.com/en-001/products/office-system-requirements-FX102921529.aspx.
Thanks,
Eben Liu