In the first instance, try emailing the current owner. You should be able to reach that person by clicking on the "Created by" link on the home page.
Of course, it is possible the address used has been abandoned and will get no response. In that case, you would need to make your case here and demonstrate, in some way, to Nabble that you have an appropriate background to take it on. On some occasions they have accepted a request to change ownership, if the person applying has a track record of posting to the forum in question.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me!