Well, your subject line is a little offensive to be candid. That script is over eight years old and been used many thousands of time by many people. It actually does work.
What you are asking for is something the script was not designed to do i.e. shoot just once a day. Likely because the author did not think of providing that option.
But the good news is that it's a script. You have all the code right there and can modify it to suit your needs.
Edit 1: one simple hack (that I have not tested) is to set the daily Start Time and Stop Time very close together at the time you would like a photo taken every day. Set the shooting interval to just slightly longer than the difference between your start and stop times. That should give you a single shot per day. Example : Start Time : 11:59 Stop Time : 12:01 Interval 2 minutes.
Edit 2: rereading your post, it appears you might have a more basic issue. Literally. Did you save the script on your SD card with a .LUA extension ?
Do you see this on your camera's LCD ? :
