You need to make a init.sqs
To do this, open up Notepad and save it in:
Program Files>Codemasters>Operation Flashpoint>Users>Missions
Save it as: init.sqs
Now, write this:
"#" ObjStatus "HIDDEN"
Where # is the number of the objective.
Back in the editor, when you need to make the objective appear, write this:
"#" ObjStatus "ACTIVE"
Hope it helps