When you use a frame action, you would reference the targeted movie clip by it's instance name, and use something like...
ActionScript Code:
some_mc.onEnterFrame = function(){
// Your code here...
};
If you're setting the code on the mc itself, then right-clicking that mc on stage and hitting Actions, should open up the Actions - Movie Clip window, which is where you should be typing your script...
ActionScript Code:
onClipEvent(enterFrame){
// Your code here...
}