On page 22 of the Retrologue 2 manual it says
Any of the 127 available MIDI controllers can be used as modulation signal. You can select the MIDI controller from the corresponding submenu.
However neither MIDI Controllers nor Quick Controls (pg. 23) appear in the popup menu as a source in the Modulation Matrix. Every other source described in that section of the manual is available. How do I get 'em to show up?