Sometimes you want to make an area clickable, say part of an image or you have some animation that you do not want to have to convert to a button. What you need is an invisible one.

First draw a square (or whatever shape you want) onto your page. Double click the shapes border and delete it. You don't need it and it makes the button look weird when you resize it.

Now convert the box into a button. Once you have done this double click it, and add keyframes for up, over, down and hit like you would any normal button.

Go back to the first 3 stages (up, over, down) and delete anything in these frames. The only thing you should leave is the box in the hit keyframe. Now go back to editing the movie (Ctrl + E).

If it worked the button should now appear as a half tranparent blue box. And if it does you have yourself one invisible button. You can now resize this to any area you want it to cover.