I have never used a Mac computer. But it is my understanding that the DNG converter will only work on folders, not individual raw files. That is a different behavior from how the converter works on a PC. So try to point to the converter at a folder containing raw files rather than to the raw files themselves.
The Mac DNG Converter does work on individual files if, in the Finder, you drop the individual file(s) on the DNG Converter's icon. Otherwise you are correct, individual files are grayed out or not shown within the utility's Open dialog box.
Wo Tai Lao Le