How to use "startDrag" (Text Masking: Follow Mouse)

I am learning to flash ,and found thing I could not understand it very well.

I made a Text masking:following mouse, just two layers totally,in the first layer, I

input some words , and then created the second layer ,but I defined the second

one as Mask layer,and draw a circle on it.of course I converted it from picture to

MC named Mask. at this point, I faced the troubles, when key the codes "

startDrag (“mask”,ture") into the second layer,it works, but I thought it should

work in the MC, I mean I right clicked the MC-mask,and then chose the action

and also input the codes ,but I got the warning like this "this code must exsit in

the on/onClipEvent ",and ok , I just add the onClipEvent,but still not work.

so my question is if I want to put the codes in the Mask MC to get same

effect, is it possible? what is the difference when put the codes in the timeline

and MC?

thanks