I just finished posting an article on the TechNet Wiki for Sending an X12 Message over AS2 and Receiving an Async MDN in a two-machine scenario. However, because the version on the wiki is not very printer friendly, I am attaching the same document in MS-Word. Here with this blog post. Over and above this, I am also attaching the MSIs that exported from both the partner machines.
All these (the MSIs and the word doc) are part of one single zip file.
Note that the MSIs are very specific to the scenario that I have used for the wiki post.