iSync does nothing with MP3 player, you have to have software for mac with the MP3 player, though I think some just mount as a hard drive and you just copy over the music, or you get a iPod to use in iTunes
Most mp3 players i have seen will just mount as storage devices on Mac. and they will on windows
if you have an mp3 Player on windows, find out whether it is seen as a "Removable Disk" or something different... If it is Removable disk, then don't worry its highly probable that it will just mount as a drive on Mac... as most mp3 players are sold with "crappy" software that is just a way of transferring music accross for those who are not really with computer technology!!
I think that its rare that you will find an mp3 player that Mac cannot interface with... but i am totally Apple so I believe in the iPod and iTunes!! Wouldn't be without it!!