I could be wrong but all aircraft added to CFS3 will automatically appear in the CFS3 UI menu. There's no need to add them anywhere if you just want to fly QC. Just select the aircraft and the enemy aircraft of your choice and click 'Fly'. It could be that the list of aircraft for a particular country is longer than the UI can display. Use the orange-red arrows next to the list to scroll down.
To add an aircraft to CFS3, simply place a folder with your files in the 'CFS3/aircraft/'-folder. If there's something wrong with it, CFS3 will tell you that that particular model cannot be loaded.