"The waiter works in the restaurant."
Translation:Kelner pracuje w restauracji.
Check this out: https://en.wikibooks.org/wiki/Polish/Feminine_noun_declension
That said, since it's a locative, it has to be "w restauracji". You would use "restaurację" in a sentence like "Lubię restaurację, w której pracuję" (I like the restaurant I work in).