Do you mean that the game is missing addons that you know for sure is in your addons folder?
Or do you wonder what "missing addons" mean?
The first problem I cannot help you with. The second problem is because the mission requires a user made addon, that needs to be installed into your addons folder before playing missions using that addon.
I hope I didn't offend you by explaining something you think is bleeding obvious, but your question isn't very clear (or I'm getting too sleepy)
Try to be more specific. What does the error message say when you try to connect to another computer?
For troubleshooting and installation problems, the best place to ask questions is the official forums:
www.flashpoint1985.com