FOR BEST RESULTS, USE AT LEAST EMACS REVISION 110135. You'll need to build from source, at least until Emacs 24.3 is officially released. Earlier revisions contain a nasty bug that can cause your Emacs to segfault when using auto-complete.
Auto Complete Mode is distributed under the term of GPLv3. And its documentations under doc directory is distributed under the term of GFDL.