XO shows the scenery package, but the airport is missing in the Airports tab

There are some (payware) scenery packages with airports that are mistakenly labeled as 2D, while they are of course 3D. This probably happens when the default airport data from the X-Plane database is copied without changing the label to 3D. XO by default hides 2D airports, but showing them is easy. Just uncheck the hide 2D checkbox:

As the airport will also be shown as 2D in X-Plane, you might want to change the label. This can be done by opening the apt.dat file with a texteditor (like Notepad++) and change 2D into 3D for each airport in the file: