This happens when the system font size is set larger than normal. I think the contents of the DNG Converter window expand with the font size, but the frame around the window does not so if the contents are bigger the lower portion gets chopped off.
To gain access to these buttons, set your system font size back to the normal size.
You are right! I set the font to its normal size and now I have a complete display of the window's content. Thank you very much for your help!