You should have an Adventure card for Perils of the Lost Coast, as well as three Scenario cards for that adventure, namely, Brigandoom!, The Poison Pill, and Black Fang's Dungeon. Double-check your Adventure and Scenario cards to see if you overlooked it.
If not, you might check your set against the deck list on this page to see if that's all you are missing.
You can return your set to where you bought it for a new set. If you bought it from Paizo, you can open a thread on the customer service board and they'll send you the missing cards.
It happens from time to time, unfortunately, though I'm not sure anyone that doesn't work for Paizo could really say exactly how often. Potentially one of the wrapped decks was dropped when the cards in your box were being assembled together.
Like elecoderduce said, the quickest way to get a complete box in your hands is to return it to where you bought it if you can. Don't worry, they get to send it back to their distributor and it makes its way back to Paizo, so no one is on the hook for the incomplete product.
If that isn't feasible (or if you bought it direct from Paizo), then contact customer service. They are pretty awesome and I'm sure they will get you the cards you are missing.
Good luck on your unfortunately delayed adventure. When you do get to play, I hope you enjoy it, because it is a great game.