Most obvious reason for not popping up would be that the workbook has no
macros in it.
If you've just changed the security setting, you must close Excel and reopen
it for the change to become effective.
If you're using Excel 2007, there are 'trusted locations' that if your
workbook is placed into, macros are always trusted and you get no warnings
when opening workbooks from there regardless of your settings.
Remember that the security settings do not go with the workbook - they are
local to the computer/copy of Excel on the computer. So if you have your
security setting on Medium to get the warnings and have a workbook with
macros in it and send that to someone using another computer, it is THEIR
security settings that rule, and if they have them set at LOW (really unsafe)
then they will not get the warning popup.
