MTP is the way nexus lines of phones connect storage to computer, and now One X (with jb) as well. And since XP is kinda old it doesn’t have support for it, and mac is just being mac so you need to download things to get it to work.
This is of course how HTC 8X and such connect as well, being a windows phone device. To use 8X on a mac you need this.
I haven’t tried the xp solution myself since I don’t have an xp computer running for the moment. Same goes with mac, so do tell me what works and what NOT works.