"Don't look out of the window."
Translation:Nicht aus dem Fenster sehen.
Because if you're addressing one or more than one person in a formal imperative way the pronoun must be explicit: "Sehen Sie nicht aus dem Fenster!". This article does a pretty good job summing up imperative forms: http://www.deutsched.com/Grammar/Lessons/0109imperatives.php
If you were trying to use the imperative form, I believe the answer should be in the form luca.anceschi posted above.
luca's post (as I know) is imperative form for informal you, so wouldn't "sehen nicht..." be imperative for formal you?
No, you need the "Sie". The imperative for the formal you would be "Sehen Sie nicht aus dem Fenster!". Schauen Sie nicht aus dem Fenster is also correct and actually works better in context than "sehen". The imperative for you (du form) informal would be "schau nicht aus dem Fenster"
small hint: "Nicht aus dem Fenster sehen" is an universal directive, for the case you cant predict who you might address, i.e. on a sign or warning label (negation first). If you address people personally, use the taught imperative (verb first).