To mount a disk image created with dd do as follows:
mdconfig -a -t vnode -f drive.dmg
After that you should be able to see mdXs1, mdXs2… under /dev if the partition table was correct. To print the partition table run fdisk mdX.
You can now mount the different partitions/slices using mount command:
mount -t msdosfs /dev/mdXs1 /mnt/dos