Ed, the thing to do is to go to your local Mac dealer (or if there is one, the local Mac users group). Modems for Macs are about the same as those for Windows machines--it's the driver (software) and perhaps the cable where the difference is. One concern is whether their Mac has enough memory to run a web browser and enough hard drive space for the program and any downloaded programs and data.

--some only moderately informed opinion from seed