Dominica has two modest
airports
, Canefield and Melville, the former being much closer to Roseau, the National Park and the dive centres south of the capital. When booking your ticket, you can request Canefield as your destination - LIAT's tickets denote the respective airports with C (Canefield) and M (Melville) after the arrival/departure times.
The main carrier, LIAT, is hardly reliable, however. Locals refer to it as "Leave Island Any Time", since it's often up to two hours behind schedule. You should definitely re-confirm your onward flight to ensure that it's still scheduled