The idea is to use the EVALUATE topic, not the COMMAND one (type /help /dde
and read the entire page, if you haven't already done so).
However, to have mirc automatically notify your program of new channel text, you need to set up an on TEXT event in mirc, something like:
on *:text:*:#yourchannel: dde <name> <topic> "" $1-
where <name> and <topic> as defined in your app.