XBMC has AFP (Apple Filing Protocol) client support for media on all platforms. We don't recommend using it, as AFP support in XBMC is very buggy right now.
Known issue 50% Of the users are reporting Crashes when using AFP. This mostly is detectable by seeing something like "1174.20:17:33 T:145559552 ERROR: LIBAFPCLIENT: I have no idea what this is a reply to, id 29249." at the end of the xbmc.log.
The Library we use for AFP is not maintained by the developer anymore. And we are not able to fix bugs in it ourselfs. We are already looking into a replacement for supporting AFP with another lib but there is not much at the horizont atm. Users who have crashing which AFP are advised to switch to another Network Protocol (e.x. smb or nfs).
Beware. When sharing an external drive via AFP from within MacOSX Snow Leopard or MacOSX Lion keep in mind that only HFS+ formatted drives are shared. This is a restriction from Apple. You can't share FAT32 or NTFS drivers from within MacOSX! See: https://sonos.custhelp.com/app/answers/detail/a_id/505