Hi dustytrails,
Welcome to the forum. Most of what you need to know to use the mission editor, and to save your missions, is covered in the Mission Editor Tutorial, which you can download from here:
http://www.shipsim.com/ShipSimForum/index.php/topic,2886.0.html (http://www.shipsim.com/ShipSimForum/index.php/topic,2886.0.html)
Specific to your question: You need to specify a title and a starting player vessel in the [Mission] panel.
You must put at least one letter in the Description: box.
Then click File in the upper left corner of the screen, and click Save mission in the drop-down list.
Type your user name—dustytrails—into the box, and then click the Save button.
Do not click the Upload mission unless you have a complete mission that you wish to submit to the CMMs for approval. It is not necessary to upload a mission in order to play it.
If you have any further questions, you can send me a PM.
Good luck,
Marty