You'll need to know which specific bgl contains the airport. One way to do that is to have FSNav installed and then hover your mouse over the airport in its map. Among the info given in the popup box will be the path and filename for the bgl that contains the airport.
Once you know the filename, click the "Open" icon in AFCAD and click the "From Specific File" radio button, then browse to that bgl.
Come to think of it, can you have more than one installation of FSNav?