If you aren't going to buy a Mac, you'll have to steal (or borrow) one. You'd want to get access to a Mac anyway to test your solution for platform specific issues before you release a run time anyway. Generally, a file created on a windows machine will look and run quite well on the Mac. It's actually less trouble to adapt Windows Created files to mac use than the opposite due to differences in how they display text, but there still can be differences so you need to test it out to make sure you aren't delivering a headache to your Mac users.
FileMaker Advanced has to have access to the specific mac application file in order to create the "crippled" copy included in the folder with the other parts of the run time package.
You could find someone with a Mac and a copy of FM advanced to take your file and complie the runtime in the Mac format.