This problem is typically the result of interrupting the driver installation process during pairing. If this happens, remove the mouse and re-pair the device. Make sure you give the driver update process enough time to complete.
If you have Windows 7 you should be seeing the installation process spinning away on the Task Bar, then Bluetooth configuration will run on some machines depending on their specific BT driver, and then the mouse will be ready to use. This process can take from 30-60 seconds to complete.
You may also need new batteries.