Sounds like you have the 8speed Campy free hub... in which case, the Campy 9/10/11 cassette won't fit without swapping out the freehub itself. There used to be a replacement part available to convert, but this is no longer the case. I know there are other methods, but I'd have to search for them. Honestly, I'd just find a new hub/wheelset... either Campy 9/10/11 or Shimano 11 compatible would work with the respective 11speed cassette.