On your Frame (click on Frame, press F9, paste code), but if you want the countup to show in a textfield, then use the Text Tool (T), open Properties Panel [CTRL+F3], and change Static Text to
Dynamic Text. Create a textfield, select it, open Properties Panel [CTRL+F3] again, and under Var(iable Name), type,
count_txt. Then, use this code on your Frame:
ActionScript Code:
startTime = getTimer();
onEnterFrame = function(){
count_txt = Math.floor((getTimer()-startTime)/1000);
}