Not sure, did you INIT the card on MyBIOS first?
That will write a fat32-entry in the MBR and configure the rest of the space for MyBIOS-IDE usage.
After that you can format the fat32-entry on PC (do not know anything about mac) and select fat32/quick.
On a 4 Gig card, fat32 is about 1.8G (roughly half the size)
Then you can start copying the files needed to this drive.
Safely remove the CF-card (Mac will have this option too I guess)
You should be ready to go with the ATARIMAX fat32-loader.
I have folders like
R16
ROM
XEX, subdevided in LETTERS
OS_OLD
Works nicely
Later,
Sijmen.