The official ALPGEN reference is
M.L. Mangano, M. Moretti, F. Piccinini, R. Pittau, A. Polosa, JHEP 0307:001,2003,
hep-ph/0206293
The up-todate documentation is available from the web page
http://cern.ch/alpgen
, as well as from the file DOCS/alpdoc.pdf in the ALPGEN package