Hi tigris,
You need to have all of the ships in the mission, including all static and AI ships as well as the player ships
Some missions have a dozen or more ship types. You may need to have installed New Horizons, Jumbo Javelin, Elbe, and Furie in order to play some missions. You may also need ships available only in SuperPacks. If you got the mission from the Creators Forum, the list of necessary ships is usually posted there. Otherwise you can send a PM to the author of the mission and ask him.
If you download the Missions Manual, it explains how to unlock and open the mission in the editor to see what ships are needed.
The Missions Manual is available here:
http://80.95.161.114/shipsim/forum/index.php/topic,2886.0.html (http://80.95.161.114/shipsim/forum/index.php/topic,2886.0.html)
But you must register your license key in your forum profile in order to access it.
Regards,
Marty