make sure your waves are 16bit/44.1khz not zipped or anything.
are you trying to copy them to the cfcard or to the harddisk (if any)?
try it with just one sample and copy that straight to the root folder, than try and load this file. if this is not working either your problem is outside the mpc or you messed up the filesystem somehow. if it is a hdd, did you format it with the mpc? did you trash the mpc icon before cutting the connection - if not maybe the drive is still mounted in macos and the mpc has no access.
peace jo