ActionScript 2.0 :: Invisible Button Action Script Plays In Reverse?

I'm using Action Script 2 and FlashMX version 6.I have a sound volume slider that I am trying to make visible on rollover and otherwise invisible all other times. The action script I'm using is:

slider._visible = false;
on(rollOver){
slider._visible = true;
}

[code]...

When I try to put the script on the instance of the button, the action window tells me actions can't be applied to it.When I delete the script, it still is the same problem...invisible on rollover...but the output messge is this:

Scene=Scene 1, Layer=Inv_btn, Frame=1: Line 1: Statement must appear within on handler
slider._visible = false;

I've tried to put the actions in a separate actions layer but sill have the same issue and the same output as when I put it on the invbutton layer.I want to use the sound slider to control the volume of the music in a flash animation.