Memory Map are a bit behind the times and don't produce a version for the Mac which is a real pain for people like me who had built up a collection of loads of maps for that programme on my PC. I have now managed to install Memory Map on my Mac without having to go through the hassle of putting a whole new (microsoft) OS in my Mac.
I installed a programme called CrossOver Mac (http://www.codeweavers.com/products/cxmac/) for £25 and this allows you to install any Windows application and then run it straight out of Finder, rather than through a new OS - no Windows licence required.
I downloaded the most recent version of Memory-Map from their website for free, installed it and it now allows me to use all of my old Ordnance Survey / A-Z / CAA maps. I have heard that sometimes the older Ordnance Survey maps don't work on the new version, as they want you to buy the new editions, but mine work fine.
Thought that I would pass this on as I have heard a lot of people complaining about not being able to do this without installing Windows on their Mac, which no one really wants to do.