With Mission Builder open, click on the "Load Mission" icon along the tool bar on the top left.  It's the icon next to the "Create New Mission" icon. 

This will open a window that lists the titles of all of the missions in your MISSIONS folder.  Simply scroll down to any one of them, click on it, then click on "OK" to open the mission.  You can then study the map, and look at the triggers and events associated with the mission.

The default window opens to show missions for all nationalities.  You can change that to show missions for only one natonality or sort by mission type. 

When you click on a mission the file name for that mission will appear in a window near the bottom.  This is the name you will find the mission listed under in your MISSIONS folder.  If you are new to mission building I STRONGLY recommend that you make a copy of this mission file before you open it in Mission Builder.  That way you won't make any changes and accidently overwrite the original mission file.