Instead of editing the file /etc/crontab, it's easier to run the command crontab directly.
First of all see the current crontab jobs and remove them all.
To list all crontab jobs :
and for removing them all
Now type in the command line
and type your cron job, it will be like
5 11 * * * env DISPLAY=:0.0 /usr/bin/totem /path_to_file