Displaying page execution time using devel
Enabling the 'Display page timer' in the Devel settings should be enough to add it to every page. You could check:


That you are logged in as an administrator when viewing the pages and that you have got all the Devel permissions assigned (People > Permission).
Have a look at the source of your site, Devel appends the time to the very end of the page, after the closing HTML tag. It might be hidden by your browser or theme.


Otherwise have a look at your recent log messages (under reports) or your server error logs to see if something is going wrong.