Zadar is not a really convenient airport if your destination is Medjugorjie. There are no direct buses. After having cleared customs etc you'll need to get to the Zadar Bus station, there you'll need to take a bus to Split (about 3 hours, buses go about every hour or so) and then in Split one to Medjugorje (between 3 and 5 hours... a few times per day.)
You could rent a car, but then you'd have to find one you can take in to Bosnia.
Timetable info here:
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