What advice to give MAC users of my Windows freeware?

I have been distributing my Windows-based (VB6) educational freeware from my website for several years (www.dcproof.com). Several MAC users have written to me saying that they would like to be able to run my program on their MAC. I know nothing about MAC's. What should I advise these potential users about Windows emulators, etc.? (I guess it depends on which version of MAC.)