The MMB is not a scroll button, but can be made into one with XMBC. Please note this application is outside the subject purview of this community and any support must come from the XMBC community (there is one).

When I press my scrollwheel (my mouse’s equivalent of the middle mouse button) and move, it does not scroll. But maybe with a true three-button mouse, it would behave differently. However, my scrollwheel properly scrolls as I expect, and grabbing the scrollbar and dragging it scrolls smoothly.