I know bugger all about macs either :-P
But can you not do a format with the card plugged into your Mac? As you would a floppy?
i.e. On an XP PC just right click on the card icon on your desktop and select format. If there's an option for FAT under the types of format - pick that.
My XP PC doesn't have a FAT16 format option but it does have a FAT format option and that works on my 1200 Tower :-)
Dave G 8-)