This site is being deprecated.
Please see the official X‑Plane Support page for help.
You can try multiplayer with the demo, but you will be limited to 15 minutes of flying time and the KSEA area. You'll need to purchase the full sim to get rid of those restrictions.
Multiplayer is set up in the Net connections screen. You'll need to have all players on the same network, or use a third party service such as VATSIM.
Here is information on how to add aircraft to X-Plane. If there are additional steps, the aircraft should come with more explanation.
This site is no longer being actively maintained.