What?
Well I don't have that executable, but /dde mIRC command "" msg #he hello has all the required parameters, this command cannot throw the error you gave. One thing is that you didn't use /msg, meaning that mIRC will put "msg #he hello" in the editbox and hit enter, resulting in "* you are not connected", if you are not connected, for example. So use /msg there.
In which situation do you get ERROR: Invalid command line switch '/msg' ? from the ddecmd executable?
