What we basically want to say is if collision is true, then play the target movie clip, without any else needed in this simple instance. So with the onClipEvent selected to make sure our if statement will appear inside it, then drill down to Actions->Conditions/Loops->if and double-click to insert it. Your code will appear as follows:
onClipEvent (enterFrame) {
if (not set yet) {
}
}
The red highlight is telling us that we need to insert a parameter inside those parentheses or this script wont work at all. But before we do that, lets talk about what we just told Flash in basic terms:
Every time that the current movie clip enters a new frame, check to see if (as yet undefined condition) takes place. If it does, then perform the as yet undefined action inside the {} brackets after the opening of the if statement. Were nesting conditions at this point, basically creating a chain to narrowly control our movie clips behavior by placing functions inside of these conditions so they only take place when specific criteria are met.

