This depends on the entered_service= date in the DP file. If the date is later than the date of the first mission in the campaign the aircraft will not be selectable from the Player Aircraft menu. I've copied the full [MISC_DATA] entry for the default Wildcat which is selectable in all default campaigns (see below). Open the DP files of the aircraft you wish to use in Notepad & change the date or add the complete entry as necessary. I think that should fix it.

[MISC_DATA]
unit_family=1
category=1
allegiance=3
max_group_size=8
min_speed=240
cruise_speed=405
max_speed=607
min_alt=50
cruise_alt=5893
max_alt=8839
entered_service=11/1/40
crew=1