there are several programs that are available to add airline traffic..one
that was mentioned, project AI
which is free. projectai.com is where
you want to go. You can download airlines of your choice, afcad files (which
add parking spaces to the airports), etc..
There is ultimate traffic, which is payware (which I use) which adds just
about all worldwide airlines, which you can find at flight1.com or
the forums at http://dcforum.simforums.com/cgi-bin/dcforum/dcboard.cgi
Ultimate traffic uses real world airline schedules.
and there's mytraffic which I don't know much about.
I'd suggest going to these websites and checking them out as they are
all very good, but..take some reading to figure out just what you want.
I'm sure we can answer any questions you may have.