Whenever a network DDE function returns an error value, you can retrieve an error message suitable for displaying to a user by using the NDdeGetErrorString function.
DisplayNDdeErrorString( UINT uCode, LPTSTR lpszTitle )
{
char buf[256];
NDdeGetErrorString(
uCode,
buf,
sizeof(buf)
);
MessageBox( NULL, buf, lpszTitle, MB_ICONEXCLAMATION | MB_OK );
}