All right?

    Okay, this is pretty easy... you're halfway there already if you have it added into your Fortress Europa.xml file.
    The next thing you have to do is find your personal campaign file.  This is located within your "My Documents" folder (see picture below)... my folder is named "Mark".  Look in the folderwith the name of your pilot... mine is "Mark Kennedy".  This CMPSTAT file will open up with notepad.  Right, CFS3 will save one of these CMPSTAT files every time you save a campaign, or it does an autosave... the ones that you save with a name have a number before them, the autosaves are called, well, "autosave".
    Scroll nearly down to the bottom, and you will see a list of planes that are, and will be, available to you in your campaign.  You need to add your custom plane to this list as well, along with the date it will enter service.  Believe me, it is great not only to fly your add-on in a campaign, but also to fly against it, for add-ons will also be used as AI aircraft... one really great feature of CFS3!  And in that case you do not have to put them in your campaign file... once it's in the FE.xml file, it's in there for good.
    There are a great deal of alterations you can do within this file, including starting out your campaign in a jet, giving your squadron 2,500,000 prestige points, or making yourself a General right off the bat.  Play around.
       One final thing, you have to make sure you have the correct name of your plane in both locations... this caused me a bit of trouble when I first started doing it.  In the FE.xml file, you need to use the exact model name of the plane.  If you were adding the B-26G, it would be b_26g, as in b_26g.m3d, or b_26g.air, etc.  I believe in the CMPSTAT file you put in both the model name and the "short name"... just look at the other planes on the list.

     Good luck.

     (post back if it doesn't work)