The DM decoder was recently ported from Java to C++ and seems to work well. It hasn't been extensively tested beyond the standard blackbox tests as far as I'm aware, but there's nothing particularly unique about it so it should work on par with the Java decoder.
The instructions for integrating zxing are in the iphone directory. They only thing you'd need to do for the DM decoder is enable it. It's not enabled by default but the ScanTest test app has code that can be switched on that should serve as an example.