You can use DDE in VB6, but you may have to use a third party DDE Server to create the link. I wrote an application several years ago that interfaced with PLC's for large pump stations using DDE Server and then moved to DsData. You use the LinkTopic, LinkItem, LinkMode, and LinkTimeOut properties of your controls to access the data. At least in my old app that's how it worked.
Again, that was several years back and I am not sure if there are better ways to connect to a DDE source these day. I do know that after W2K, MS changed the way DDE is supported in their OS.
Good luck.