Reviews on the Mac App Store and iOS App Store will never show up right away. Reviews are first reviewed, to avoid spam comments and reviews with offensive material.
Your review will show up once it is reviewed and approved (assuming your review is free of offensive language and is not spam or some other nonsense).
Bear in mind, too, that reviewing an App which you downloaded by redeeming a promo code is not possible. The systems lets you proceed with the review process althought the review will never appear nor the rating taken into account.