Hello Luca,
please make sure that you are inside the source directory, i.e. aMule-2.2.2/. If you type "ls" to list the directory content, there should be a debian folder inside. Is this the case?
Edit: If you use dpkg-build (or similar, it has been far too long since I used a debian based system, please excuse) to use, you don't have to specify any option as everything as already been done by Vollstrecker when he wrote the files inside the debian folder.