I think PxT is correct in saying that the DISPLAY variable is not set. You can set the $DISPLAY in cron, or in your srcipt / application.

If you just want some quick, simple notification, you could pop up an xterm on, say, $DISPLAY=":0", with a few lines of code:

Code:

#!/bin/sh
DISPLAY=":0"
Pay_Attention () {
echo "It is time for you to pay attention!"
echo "Press {Enter} when you are done..."
read _dummy_response
  case $_dummy_response in
     *) exit 0 ;;
   esac
}

xterm -e Pay_Attention
exit

I'm sure there's a better way of doing that, but thats the first one that popped into my head... Heck, that as-is probably won't work, but it given you an idea of what I'm thinking, right?
Either way - the xterm will close when someone presses the enter key...

added code tags for readability --oombera