Hi hynes123,
Seriously, how you “get on Mission Editor†depends upon whether you currently have it. That, in turn, depends upon what version of Ship Simulator 2008 you have.
If you have the boxed DVD New Horizons, you should already have it.
If you have the original 1.0 version, you need to download and install three things in this order:
#1: The 1.1 update.
#2: The 1.1 Mission Editor.
#3: The 1.3 update.
Tell me what your current status is; either post it here, or send me a PM.
After you have the ME (Mission Editor), you run it as a separate program.
You should read the following documents:
The Mission Editor Tutorial in PDF form can be downloaded from the Tutorials section of FAQ and Announcements. Follow the link to find the 1.4 version.
Custom Mission Creation Guidelines explains what is acceptable in a mission. http://www.shipsim.com/ShipSimForum/index.php/topic,4942.0.html
How to download and upload custom missions in-game explains the uploading process.
http://www.shipsim.com/ShipSimForum/index.php/topic,4506.0.html
Good luck,
Marty