By default, Excel 2008 saves newly created files in its native .xlsx format,
so you shouldn't have to select it anywhere... Just save the file, navigate
to the desired folder & type in the file name you want to use.
If it's an existing file in a different format -- such as .xls -- and you
want to create a copy of it in the new format, use the File> Save As
command. Then do as above but select Excel Workbook (.xlsx) from the Format:
pop-up in the Save As dialog window before you save. Along the same lines,
if it's something other than .xlsx & you've used certain features of Excel
2008 not compatible with that format you will be prompted to Save As a copy
using .xlsx in order to preserve those changes.
It's inconceivable that the choice not be in that list of file formats. Are
you possibly looking in the Format Menu in the application Menu Bar? What
"message" are you referring to? What is the exact wording of the message &
what specifically are you doing that triggers it?
HTH |:>)
Bob Jones
[MVP] Office:Mac