Most of the add on missions I downloaded require a specific aircraft which at times I don't have. I have similar type aircraft in my collection but the title has to be exact using the correct texture. For example I need the folder for the "DHC6-Amphibian_FOPI" but am unable to find it anywhere including PAD. One other question:  How can I replace the default aircraft with another type which I already have?

Let's say a mission uses the Beech Baron and you want to change it to the Tiger Moth instead. I'm exaggerating here a bit.

Go into the aircraft.cfg file of the plane and locate the texture you want to use and copy the title.

example:

[fltsim.2]
title=

Now replace/paste in the FLT file of the mission, under where it shows the aircraft sim that the designer used, the new title.

example:

[Sim.0]
Sim=

This can be done any time you get an aircraft initialization error when starting a mission.

If you do not want to do this you can also go to free flight, choose the plane you want to use, go back to the mission and ignore the error when loading. FSX will then load the default aircraft you chose from free flight. You will have to do this each time you load the mission though.