CaptaineNemo, if you're getting the error "catalog file for domain 'amule' not found", then you need to create the package by copying a proper aMule.app application. The empty skeleton aMule.app that is in the source distribution is not sufficient.
Basically, aMule.app includes a catalog of translations of the strings in aMule so they can be presented in various languages. Your package probably doesn't contain this catalog, if you're creating it yourself. That seems to be what the error is complaining about.
If you prefer to know exactly what's going on, you can read the mac_packager script in the source tarball (in src/utils/scripts). In fact, a little judicious editing of that script would allow you to create an aMuleGUI.app from scratch in exactly the way that we create official aMule.app releases.