The middle mouse button can be changed.
Ensure you have the correct drivers (v8.15) for your version of Windows and are in the correct spot (usually a link in the lower-right System Tray).
This is NOT a very good mouse if you play video games though. If you play games or this mouse frustrates you I strongly suggest you get a Logitech mouse.
While expensive, the MX518 may be the best gaming mouse overall.
I STRONGLY suggest if you buy an expensive mouse that you go to a store first and see how you like the buttons (especially the middle button). I really, really hate middle buttons that are spongy to press and hard to turn.