If you want to change the thousand and decimal separator in Excel for Mac, you need to change it at operating system level.
For example I have Leopard in English and Excel 2004 in Italian and I have changed the decimal separator from .(dot) to ,(comma).
How to proceed:
0) close excel for mac
1) open mac os system preferences from the dock
2) open Personal, International
3) go to format
4) select a region where the comma separator is used, for example United Kingdom. In the bottom of the popup window, the numbers should appear in the correct format
5) relaunch excel for mac