It is the polite form, yes. www.conjugation-fr.com/conjugate.php?verb=regarder
So how come Duo won't accept "Ne regardes pas dans la boîte" if "regardez" is acceptable?
In this sentence "regarder" is imperative - a command. The second person singular form is "regarde" not "regardes". It's the one situation where the imperative form is different to the indicative.
As BenYoung84 said, the familiar form of the imperative often looks like the first person singular form of the present indicative. Take a look at the link just above and scroll down to see the imperative conjugation: regarde (familiar), regardons (first person plural, the "let's" form), and regardez (the polite/plural form).
Merci beaucoup for both of your responses. When you're trying to learn French grammar by immersion it helps when someone occasionally explains something to you. :)
If you got the English to translate, then "regardez" should be allowed. Me, I got here from the audio clue, which obviously would not sound the same.