Lenovo T420 running Windows 7 64-bit. Fn+8 key combo is not working to let me access Ultranav settings. Control Panel > Mouse also does not have a tab for UltraNav, only settings for a normal mouse (though its cursor movement speed control works with the Ultranav). By the way Fn+F6 and Fn+F3 also don't work, though other Fn combos do work (Brightness, overhead light). I'm most concerned with getting at the UltraNav settings though.
Most likely you dont have the proper driver/application that supports the Lenovo specific function keys. You would need to download them from the Lenovo website. I took a quick look and it appears that model is no longer in production. I assume you recently upgraded to Win 7 and thats when this issue started. If there are no Win 7 drivers, you can try drivers for older OS, they may work.