First switch over to singlplayer by going
File>Preferences
Then near the bottom, there are two radio buttons that say "Singleplayer" nad "Mulitplayer"
Switch over to singleplayer and choose what object you want for you objective.
Then switch back to multiplayer. You may notice that the object that you chose as your objective has reverted back to "static" mode. This is normal, the same thing happens with teleporters.
By now it should work, although I have never done an objective map...
Oh, and another thing, you may want to consider using S4Mod V2 for multiplayer games, MUCH better...
"There are very few problems in life that cannot be solved by the proper application of high explosives"