The Microsoft KB details how to attach to a Microsoft Outlook folder in "ACC97: Sample Procedure to Attach a Microsoft Outlook Folder" (Article ID Q161174). I found dealing with MSIE4's Outlook Express a different matter.
Since I could not get the proper connection string, I resorted to exporting the desired folder to Windows Messaging (during this process Outlook Express should not be the default Simple MAPI client). I then attached to the folder using the following connection string:
'Source database type:
strConn = "Exchange 4.0;"
'Table name (MAPILEVEL=<storage|folders>):
strConn = strConn & "MAPILEVEL=Personal Folders|;"
'Source table type (0 for folders, 1 for address books):
strConn = strConn & "TABLETYPE=0;"
'Database name (DATABASE=<path>):
strConn = strConn & "DATABASE=" & m_dbDAO.Name & ";"
'Profile name (optional):
strConn = strConn & "PROFILE=Bryan D. Wilhite;"
'Password (optional):
The greatest challenge to getting a sucessful connection was setting MAPILEVEL correctly: "MAPILEVEL=Personal Folders|;".