Hi,

FS already comes with AI aircraft - go to Options/Settings/Traffic and move the AI traffic slider to 100% (FSX may be slightly different).  Make sure commercial and GA traffic boxes are checked, and look around a major airport - you will see them

They are called AI aircraft (yes, it stands for artificial intelligence) because they act like a plane with a "virtual crew" that operate like human beings, even though they are only created by the computer.  While they aren't very smart, they don't do a half bad job.

Hope this helps,