Make sure you have new batteries and that there are no power cables close to the mouse or receiver, I have seen this cause problems many times.
1. Press the connect button on the receiver onces
2. Press the connect button on the mouse until the receiver light stops flashing
(Sometimes the connect button is under the battery cover for the mouse)
3. If you have a keyboard which also connects wirelessly, and it is the same make, press the connect button until the receiver light stops flashing.
If the wireless keyboard and mouse are different makes, there should be a separate receiver,
(also known as a dongle) for each.
You will need to follow steps 1 and 2 for each device.