cr 7.3 or better is needed. what version do you have?
In the DNG converter interface, are you just choosing the FOLDER, or are you opening the folder containing the raw files. You need to just choose the folder, but don't open it.
Probably one of three issues:
1) The DNG Converter needs to be at least DNGC 7.3 to recognize the D600's raw files,
2) Outdated Nikon Transfer or ViewNX software was used to transfer or view the images, and you can correct this corruption by using a utility from here: Fix Corrupted Nikon NEF Images or
3) You need to single-click on the folder of raw files to select it rather than double-click on the folder to open it like most other software works: