The online help contains a book named "DDE Support", located under "External Access" within "Technical Reference". Here is a VEX file (33K) containing an example that lets you explore MSM-Workstation's built-in DDE support.
two instances of the EXE, placing their main windows side by side on your
screen. In one of them, select Action\New Window\Server. This creates an MDI
child with a textbox and a document that are published over DDE. In the
other instance, select Action\New Window\Text Client. Locate the service
beginning MSM and followed by a number. This should match the Service line
displayed in the server window you opened in the previous step. You'll get a
list of items. Select Text and press OK. In the resulting client window you
get a textbox containing the same string
as in the server's. Because the client's radiobutton is set to Automatic, when you change the server's text, the client's is updated. You can
experiment with other settings. The three buttons on the Client window initiate operations from here. For example, the client can ask the server to
send the linked data (not usually necessary in Automatic mode). It can poke its own (perhaps revised) value back to the server's textbox. And it can
submit its value to the server as an Execute instruction, which the server can accept or decline.
The Action menu (either window) lets you open an Events window to monitor the MWAPI events associated with DDE. There's also a Connections dialog that allows you to see what connections you have, and in what role (client or server).
Try the Document Client window, linking it to the Document item from the server.
See what other DDE services are advertising on your system, and what items they let you get at.