For 4GB SD card, I partioned to 2 parts with Mac disk utility, copied (drag & drop CHDK files on Part1, (FAT16 16MB partition, other partition is 3.9GB FAT32), then "made bootable" with "Cardtricks" on the PC. Then checked, it worked OK, showed 1181 shots left to take.
This is discussed elsewhere also, but I'm posting hoping you may get a clue from what I did. You'll see I used Cardtricks, that's because I was getting nowhere trying to make bootable on the Mac, but others have.