Delta366, I highly recommending both updates, 3.1 and 3.1a.
When downloading aircraft, they always come in an archive (compressed) file, such as *.zip, *.rar, etc.
You need to extract all of the files and folders contained in these archive (compressed) files.
There is usually a readme file with instructions on what to do. It is possible, depending upon the specific download, that other items need be downloaded, as well, in order for the aircraft or other item to work properly.
A specific download may have separate folders in it, other than the aircraft itself, such as weapons, pylons, guns, etc. In this case, simply place the aircraft folder (the one with name of the plane on it) into the main game folder of CFS3. The contents of the weapons folder would go into the weapons folder in the CFS3 main game folder, the contents of the pylons folder would go into the pylons folder of the CFS3 main game folder, etc.
I hope this helps.:d