So far Aperture does not support Leica M Monochrom DNG's and the best way I have found to get the files into Aperture is to open the DNG's up in Adobe DNG, export them as TIFFs and import the TIFFs into Aperture. This does the job I suppose in a round about clunky way, but you obviously lose DNG control inside Aperture.
If anyone knows a better way I would be most grateful.