Firstly we need to make virtual device for our iso image (use mdconfig for it):
howtobsd# mdconfig -a -f /path_to_iso/Mac_OS_X.iso
md0

md0 (command return) – it is name of virtual device (should appear in /dev/md0).
you can check if it appeared in /dev:
howtobsd# ls /dev/ | grep md0
md0

And now we just need to mount virtual device:

howtobsd# mount_cd9660 /dev/md0 /cdrom

That’s all, your iso image available in /cdrom directory.

If you need to unmount and delete virtual device, do following commands:
howtobsd# umount /cdrom
mdconfig -d -u md0