Invisible button is a button which only has a hit state defined, the other states are left blank. This button is used for putting the action to drop down buttons hidden when mouse reach it.
Add a new layer and named it as Invisible Button. Draw 2 rectangular shapes around the drop down button. Select the shape and convert the symbol shape (use Insert>Convert to Symbol or press F8) to Button. Double click the button and press F6 at its hit area to make the button invisible.
I'm a beginner and just learnt flash for about 2 month. I really like it. Thanks a lot to Citra and Arifin who have introduced this incredible stuff to me.