I am in exactly the same situation! was given the trackpad by my brother, who doesn't need it anymore, got it home, paired it without any problems, used it and it was fine.
Closed my 2009 MacBook, running under Lion, and now it won't recognise it at all. I tried deleting the trackpad and re-pairing it, but no dice! It won't pair, keeps telling me to make sure the device has batteries (it has, they may not be fully charged, but enough to have worked yesterday) and is on... well.. not easy to tell, have to keep trying to turn it on. In designing a sleek trackpad someone forgot to ensure the user could tell whether it is on or off!
Anyone able to help?