Open the plane's .dp file and look for a section called [MISC_DATA].  If it's missing you can copy one from one of the stock fighters such as the Corsair. 

I've found that as a bare minimum you need to have an "entered_service" date for the plane to show up in Mission Builder.  If I download a plane that doesn't have one just adding the following two lines will make it appear:

[MISC_DATA]
entered_service=11/1/35

You can add additional information such as the family, category, and allegiance as well.  Family and category are important when adding bombers so that they behave properly and so that the choice of mission type matches the type of plane.