It's your select command, you never selected mfgID, so you cannot use it in part of the ddl. The query only has information of the UserID and the Full Name as techid. So when you ask for mfgID, it goes "what the.. what do you want? Do you want UserID or do you want techid? What is mfgID?"
You're also only selecting the user information from aspnet_users. Within the table aspnet_users, do you have a field called mfgID? If so, add it to the select statement after techid:
SelectCommand="SELECT [UserID], [FirstName]+ ' ' + [LastName] AS techid, [mfgID] FROM [aspnet_Users] WHERE [isTech] = 'True' ORDER BY [LastName], [FirstName]">