There is a VB6 application that ships with the DDE server that shows how to use the DDE Server to read & write PLC data via the DDE interface, and one that shows the same process only it is using the OPC interface.
But I suspect that this is not what you're asking about. Are you looking for an ActiveX control that implements the DDE interface? If so, we don't have one of those. I've had several folks through the years ask for help in creating one in exchange for their providing us a copy of the completed work, but so far no one has followed through and given us a copy of the control to make available.