Don't worry, we'll figure that out =)
So, you need to make 2 steps.
First, you need a method that is called once in every frame. I guess you already have done that, e.g. by creating an enter frame event listener.
In this method, you will check if any of your objects collide. In your game, you will want to test if any of your enemy objects overlaps one of your tower objects.
Let's just look at one test. (In the end, you will have a loop that tests all your objects, of course.)
The 'enemy' object is somewhere in your display list, the 'tower' object at another position. But no matter where they are, you just want to find out if there bounding rectangles overlap. Thus, you first have to find their bounding rectangles in an arbitrary coordinate system (read: in a sprite).
SPRectangle *enemyBounds = [enemy boundsInSpace:self];
SPRectangle *towerBounds = [tower boundsInSpace:self];
So, here the coordinate system (sprite) I chose was 'self', but in reality it does not matter. The important thing is that you use the same object in both calls.
Now you've got 2 rectangles in the same coordinate system. All you have to do now is check if they overlap.
BOOL collision = [enemyBounds intersectsRectangle:towerBounds];
The reason that my sample above (the old post from a month ago) just used the "bounds" method (instead of "boundsInSpace") is that both objects were in the same coordinate system (they had the same parent). But that's not always the case, so the way I describe here works in all cases.
I hope that gets you started!