I’ve heard of others have problems with EDI or maybe it was you posting some other messages about it… I know very little on how EDI works inside BizTalk so I can’t help you much.
You might have better luck with a custom property. Just sure you set the context type to “Message Contact Value” rather than the default. That will make a big difference.
This might help: http://www.biztalkgurus.com/blogs/biztalk/archive/2004/09/17/Working-with-Untyped-Messages-In-An-Orchestration-_1320_-Part-2.aspx