They mean that you should set an alarm with a new stream and then set the action to night mode.
This way you could set an alarm that goes off after 30 minutes that will turn off NPR and then put your chumby into night mode.
So the steps would be:
1) Configure the "when"
2) Configure alarm time (in your case current time + 30 minutes) and date
3) * Configure the "audio" alarm sound to the new stream (http://.)
4) set the action to set the chumby to night mode
5) select (check) the "Set" alarm checkbox and select "Don't show alarm screen" checkbox
Now that the alarm is set, you can use your chumby to listen to NPR. Once the alarm time is reached, it will turn off the radio and then put your chumby into night mode.
I have a "daily" alarm set to do this exact function, so every night my chumby turns off any sound and goes into night mode.