what you are running into there is theres several kinds of .wav files for god knows what reason.
if you have any kind of sound editing program see if theres a setting to save your wave file as "PCM wav". thats the basic format that the mpc recognizes.
there are also different ways to translate .snd files into .wav but i dont have a mac so i dont know whats best for you. it is a bit tricky because there is a whole other sound format called .snd that is NOT the same thing....\\
actually what am i saying - you have an xl. if im not mistaken the xl has an option in the SAVE screen to save as WAV - whereas my 2000 classic sadly does not.