Hi,
It is actually very easy. You must download a program called TTools. It is amazing. All you have to do is decompile the main traffic bgl. Once done it will display 3 seperate text files (airport, aircraft and flightplans) To add aircraft you can either replace default aircraft by changing the name to the name of the addon aircraft or like me, you can add entirely new aircraft with their own flightplans. I use the military packages provided by planespotter. These include the necessary text info used to edit the files and also the necessary AFCAD's to allow parking at the military bases.
Hope that helped?
Eric

Mav, it sounds a lot more complicated than it is.  Decompiling consists of pushing a button.  After that it's altering or adding a couple of lines, pushing that button again, and then firing up FS to make sure it worked.  Make sure you get AFCAD too so you can add mroe parking spaces.  It also is exceeedingly simple. 

Kevin

somewhere ,there's a tutorial on this stuff..i think it's on www.jetdoc.com .
a great site,by the way! lots of tutorials and help!