objectives are numbered in the briefing if you look (OBJ_1, OBJ_2,etc.)
in the trigger or waypoint that you want to activate it and change it to a tick, put:
"1" objstatus "DONE"
or put 2,3,4 depending on the obj number.
you can also put "FAILED" to make it a red cross
also there are hidden objectives - go to the editor's depot for these as macguba says.
M