I see no discoverability option I am running Yosemite 10.10.2
The mouse shows in the bluetooth pref pane with a pair option to the right, I choose the pair option and it says connecting, then it says could not connect to the device.
Looks like a mouse issue, I have an old Apple mouse which connects and works.
The mouse that will not connect is quite an old Logitech Bluetooth Laser Travel Mouse.