Sounds like the problem is the button, not the scrolling part itself, yeah?
This code on the button should do the trick...
on(press){
scrolling=true;
}
on(release){
scrolling=false;
}
... on your text clip you need to add...
if(scrolling){
textbox.scroll+=1;//or -1 depending on the direction
}
If you need to scroll up and down just have two buttons with upScroll and downScroll as the variables and alter the textbox code to suit.