Frank - save the various variants in a folder, like you do on a PC. (i.e. each variant should have a separate folder within one global RP Variants folder.)
Use Finder, open up the Applications and highlight RealPolitik.
In the Preview pane, there should be a button that says 'More Info ...'. Click it.
A new window should open showing the various info about the RP program. Near the bottom there's a 'Plug-ins' drop-down. Open it up and you should see a list of all your current variants. At the bottom, a plus and minus sign.
Hit the plus sign. It will open a 'Find Plug-Ins window'. Browse till you find the folder containing your specific variant file. (You want to highlight the folder, not any of the files within it.)
Hit 'Choose' and your plug-in should be installed. Make sure there is a check-mark beside it in the plug-ins list - if its not check-marked it won't show up when you load RP again.
There are bugs - sometimes I used to find that it gave me an 'error - invalid season' message when loading a variant that I had just installed. Double-check the various RP files if this happens - I think its the .var file that sometimes need editing but its been so long that I'd have to think about that one.
Hope that helps and isn't too convoluted. Cheers,
Trout