Mike,
There is very limited documentation on the BizTalk 2004 EDI Base Adapter, but there is:
http://msdn.microsoft.com/en-us/library/ee276946(BTS.10).aspx and
http://msdn.microsoft.com/en-us/library/ee276975(BTS.10).aspx
I would suggest that look at using a later version of BizTalk, either 2006 R2 or 2009.
Rgds, Nick.