First you need the trigger to be activated by east not present or something to that affect. i hope your that far.
You should download this Chris' OFP Script Editor on this page
http://www.ofpec.com/ed_depot/index.php?action=list&game=OFP&type=to&cat=xyz or look in Editors depot for a tute on mission making.
its kinda hard to explain and you could just be doing one little thing wrong, so i added the below
"1" objStatus "DONE"; hint "You have a new objective. \n\nCall for evac." Or something like that, the “hint” makes that ding sound you here when the objective is done. After hint you can right “Good job soldier!” or what ever, that pops up in the left top corner. Now in you’re your briefing you should have something like this
<!-- Objective 1 -->
<p><a name="obj_1"></a>
<!-- [OBJ_1] -->
Clear town
The part that says “Clear town” is the objective. Witch is obj_1 .
someone will add to this post iam sure and help u out better then me

and

to the forums, cheers Schuler