Microsoft’s Mac IntelliPoint drivers, used for Microsoft USB mice and trackballs, are not compatible with OS X 10.11. Microsoft does not plan on updating them, but they have posted a use-at-your-own-risk workaround. I personally would recommend using USB Overdrive or SteerMouse as an alternative driver.  http://answers.microsoft.com/en-us/windows/forum/windows_other-hardware/el-capitan-and-microsoft-mouse-intellipoint-not/ff530983-775c-4303-9a92-4ed17f7c1896?auth=1