Damn, seems like there is no way to change it the Mac version of Excel
Changing Formats in the International System Pref pane has no effect on Excel.
If all you need to do is diplay the numbers with the comma separator, I've put together a simple
formula that will do the trick. The way I have set it up here is to display 2 digist after the comma, but you can easily customize it to suit your needs.
=
FLOOR(A1,1)&","&MID(TEXT(ROUND(A1-FLOOR(A1,1),2),"0.00"),FIND(".",TEXT(ROUND(A1-FLOOR(A1,1),2),"0.00"))+1,2)
A1 above could be any cell reference