Your problem isn't with the HID bluetooth profile, your problem is with pairing a bluetooth device with your N900. You should be able to pair the mouse with the N900 just like with other devices, entering the PIN code, etc.

It probably just has to do with the order that you're doing things. Just keep trying...

Can you pair other devices, like a keyboard?

(EDIT: I have successfully paired my stowaway keyboard with the N900 after a few tries, but I haven't tried a mouse.)