I need to know what model mac you have. This would help. Also what operating system you are using. Older macs may not be compatible unless you update the system. The latest macs with the intel chip work fine with this. Once quick solution is to update your software to the latest versions by going to your apple with your mouse in the upper left corner of your screen and then clicking it. Scroll down to the title software update. click that and then click update now. Check the boxes of all you want to update and then hit enter. *Note: you must be connected to the internet in order to make these updates. I would suggest being connected something faster than dial up.
After you have done your updates, try to reconnect the Marantz. Remeber to connect it first then turn it on.
Another solution is to format the card on your mac first by using a card reader and the program Disk Utilities located in your utilities folder. It is important to choose the Fat 32 format (You will see this in your pull down menu). Note: This will erase everything on your card.
If you have a card reader, you can always take the card out of the recorder and try to read it that way as there may be a communication issue with the mac and the recorder, but if it can be read on the PC and not the mac, I doubt that is the issue.