The pairing code for the Dell Bluetooth Mouse #: M-RBB-DEL4 IS "0000"
I have had a problem getting mine to pair (I have 2 on different computers & have the same problem with both). It will say successful pairing but does not work.
I have found that the easiest way to fix it is to re-pair it. But sometimes that doesn't work & I have to do it a few times.
I then noticed that when I go to pair it, in the "Add Hardware" dialog, it sometimes shows "Dell BT Mouse" or "Dell Mouse" or "BT Mouse" or "Bluetooth Mouse" & sometimes it will detect it before it's put into pairing mode.
From my experience whenever it detects it before it's in pairing mode it will not pair successfully. It also fails most of the time for all others aside from "Dell BT Mouse".
Now I turn it off & on until it shows up only after it's in pairing mode & has the name "Dell BT Mouse" & it works fairly consistently .