the problem is you have only one hd.
if you had 2 hd's you would have no problems.
you see mac sets the hd to be a GID type disk as opposed to a fdisk style disk that windows and linux use.
what you could try is this:
boot into OSX open disk tools.
if the main partition for OSX takes up to the end of the disk then you have to shrink that partition to leave you enough room for mint (say 10GB).
then you create a swap partition in the free space that is 1.5x your RAM.
then you create a fat32 partition with the rest of the free space.
now insert your mint cd reboot and use the alt key to choose the CD to boot off.
install, at the partition stage set the fat32 partition to be / and format as ext3. you can format the swap as swap again if you like.
take note of the / partition dev name (eg /dev/sda4)
at the last step use the advanced button and set it form (hd0) to /dev/sda4 (the / partition)
now when you boot you have to hit the alt key (insert) and choose the "windows" disk to boot into mint.