Hey Peter, thanks for your post & welcome to the Community!
Congrats on the new purchase - takes a little bit of time to get used to the differences of the Mac Ecosystem. Once you do, not many look back. Our desktop products natively do not run on Mac, as they are created for the Windows ecosystem.
The best way to get Reckon Accounts working on a Mac is through Apple's Bootcamp or third-party tools like Parallels. These programs allow you to install Windows operating systems as a secondary OS... essentially creating a virtual machine.
Doing this will allow you to do one of two things;
Install Cashbook V8 - If you install an older version of Windows, which is compatible i.e. XP.
Upgrade to Reckon Accounts Home and Business 2015 if you install a newer version of Windows i.e. win8, wi8.1 etc (to get your datafile compatible you would need to do a step upgrade (See Page 12 of upgrade guide >>)