i think you will have to find out which ISP owns the address. probably need the cops
then you will have to get the cops to get a list of people who logged in using that IP in a certain period.
you then have to tell them the time that they were logged in.
the problem is the first step. after that it is easy.
i gather you have those programs that help you track macs now installed? did you have anything like that installed on your onld imac?
go get you mac back!!! good luck.