start-dde-server creates an instance of a server of the class specified by name which then starts accepting transactions. If successful the function returns the server, otherwise
nil is returned.
You need to call
start-dde-server in a thread that will process Windows messages. This can either be done by using
capi:execute-with-interface to run it in the thread of an application's main window (if there is one) or by running it in a dedicated thread as in the example. DDE callbacks will happen in this thread.
LispWorks User Guide and Reference Manual - 13 Feb 2015