Ok, this is what I want you to do.
Check the tiny light and make sure it is on.
If there is no light make sure the switch is on, if it is on and still no light change batteries or make sure they are inserted correctly.
If all checks out and you have used this mouse before with this Mac put it down and click the clicker once to reconnect.
If the above fails then, open System Preferences found in the Apple menu.
Now click on Bluetooth.
Make sure that Bluetooth is turned on and discoverable.
Check the box for Show Bluetooth in menu bar.
If the mouse does not show in the list slide the on/off switch on the mouse off and the on again. It should now appear in the list (give it a few seconds).
Click on the mouse in the list once to highlight it and then click Connect.
You should now be able to use the mouse.
Quit System Preferences and you are done.