Yes you could get a taxi, and might be more economical if you could share one with others at your hostel or lodging. I don't recall the name of the bus terminal but it was not one of the larger ones. I think it was the "yellow bus" but best to ask. Perhaps just drop into a hostel and inquire. IF I recall correctly, the bus comes to a divide with a sign to Las Penitas to the left, but goes straight. But just goes a short distance and then turns around and heads the direction you want. Perhaps tell the bus driver you want to go to either the PLAYA ROCA or Casa de Patricia (a little past Playa Roca) and that should help.

KPR77 gave a very thorough answer so I won't add to that other than to say that while my spouse and I have been completely comfortable on the local buses in 4 months of living here, doing the negotiations to get from the Managua airport to Las Penitas sure would be easier if you had some Spanish. And you will also want to be a person who is comfortable with perhaps being totally crammed into a tight space, as they really load the buses up. If you were travelling with big suitcases, going by local bus would be quite uncomfortable and even maybe impossible, as we've been in situations where the overhead bins weren't big enough to hold our backpacks, which we had to put in our laps instead. A wheeled suitcase in your lap would not be quite so easy! Also, the place where the buses pull in at Leon is a bit of a crazy scene - safe, just crazy - and that might be intimidating for travellers who aren't used to Central American bus travel. As for shuttle options from hotels at Las Penitas, we spent a very happy week over Christmas at the beach but saw no hotel that would be of a sufficient size to be sending shuttles all the way to Managua to get people. However, there are private shuttles that you could hire to take you from the airport to Las Penitas, but they definitely won't be as cheap as the local buses. I see Tierra Tours has daily shuttles between Leon and Granada, so maybe they're an option, as Managua is in between those 2 cities. TripAdvisor won't let me give you the URL for Tierra Tours, but if you google "shuttle nicaragua" you'll pull up their site and others. But we've never used a shuttle here, so I'm not able to recommend a specific one.