On 2008 there is NO such listing under tools and putting "fixed
decimal" in help returns "no results.
Ayone else?
Options in Excel is called Preferences on a Mac and should be under
the application menu. That is the menu called "Excel" in this case.
Also usually accessible with "CMD-,". That's the Command key (also
known as Apple key) and comma.
I don't have 2008 and don't trust Microsoft to match conventions, so I
can't be sure of all this.