There's a lot more to it than you might think. Got some good advice and bad advice, but unless you have the knowledge already there's going to be some trial and error, especially when Linux is involved, the sytem that "just doesn't work."
I got it to work by installing Refind via the command line. After getting the wrong advice, I found out how to do it correctly. Refind is a boot loader that gives a startup screen allowing you to choose what operating system you want to boot. I don't see why I had to dual boot. I thought I could simply install another OS, then go to startup disk whenever I wanted. But that didn't work. Nor did trying to install the Mint CD from an external CD drive. I had some trouble getting the Mac to work at all after that. Brief panic.
Getting it to work: I partitioned the hard drive using MSDOS (Fat). Per advice, I also made a swap partition, also in MSDOS (Fat), the reason being I only have 3 (or 4) GB of RAM in my Mac.
As I have come to expect from Linux, the Mint installer isn't obvious. Given a list of partions on my drive, in not the clearest manner, I installed Mint (from my internal CD drive) into a partion using EXT4 journaling file system, selected the mount point (slash, or /), changed the swap partion to EXT4, and selected Device for boot loader installation, which will be the same name as the new system. Sort of. It should be whatever you named your new parition when you created it (like Mint or Mint partition) but it shows up as sda4 or sda5 depending on what you already have on your drive. Some new alphabet soup for me.
Then you get to spend hours in the update manager. You don't have to, but it's a good idea, esp. if you want to run the latest version of Firefox. As in Ubuntu, the update manager is quite finicky. Lots of trial and error, trying different mirror sites, lots of waiting and watching unrealistic progress bars, trying to deciper the OS's instructions and responses, trying to figure out what oddly named files are for. Freezes and restarts for no apparent reason.
Scolling through and viewing the screensaver choices froze my computer. Then when I was gone, and the default screensaver kicked in, it froze my computer.
Installing the OS was slow. Startup is slow. Downloading is slow, when it works. You're expected to know the purpose of enigmatically named files. A typical Linux experience. It's a fiddly system that most people are better off leaving alone. On the plus side, the interface is attractive and snappy.