Scrollbar clicking

I downloaded the scrollbar file and the code for mousewheel operation.

The file working okay. The only problem for me is the mousewheel operation is only active if the movie is clicked once.

I thought I can have a code telling that “click left mouse button when a cursor is on the movieclip” then the mousewheel works without actual clicking. Unfortunatelly, my actionscript knowledge is very limited and i couldn’t able to find any answer in the forum.

is it possible? or any other ways to solve it?