Please be more specific.
You will need to go through your specification and tweak an existing packager or write one from scratch based on field definitions in the spec you possess.
You will need to determine from your conversations with Mastercard , if the messages will use ascii or ebcdic and what type of channel to use. If a 2 byte length header is used, use NACChannel in your channel configuration.
At the end of the day, you will need to do the configuration together yourself with possible help from the jpos user group if you can't figure something out.
-chhil