Just to clarify for the couple of people who are struggling to install this plugin (me included..)
It DOES NOT install via the Preferences/Plugins/Add Plugin dialogue.
(that was my problem: installing on a new PC, and over-familiarity of that dialogue, and not RTFM...)
RTFM on the first page:
1. Unzip HDCD.zip to put HDCD.dll into the Plugins folder
2. Go to Preferences/Player
3. Press the 'Equaliser & DSP' button
4. Tick the 'HDCD Decoder' in the 'DSP Plugins' section of the dialogue
HDCD tracks will now trigger the HDCD plugin to expand to 20bits