Scroll bar showing? Just check its alpha. If it's 0, then it's not showing.
When in doubt, look at the controls that come with NGUI. Both the background and the foreground need to be linked to the UIScrollBar as "Background" and "Foreground" fields.
Check your panel. If you turned on its static flag, then there should be a warning on it explaining to you that moving widgets underneath the panel will have no effect. That said, why are you using TweenPosition? If you have it linked as a scroll bar, when you start dragging it, the scroll bar's logic takes control of it, and your tween position won't work, as the two of them will fight for control of its position.