Have you edited the config.txt file to enable the DAC+?
RuneOS kernel settings: read "I²S kernel modules"...
http://www.runeaudio.com/documentation/runeui/settings/#runeos-kernel-settings
config.txt: Read down to "Device tree"...
https://www.raspberrypi.org/documentation/configuration/config-txt.md
Device Trees, overlays and parameters: Read down to "#dtoverlay=hifiberry-dacplus"...
https://www.raspberrypi.org/documentation/configuration/device-tree.md
Read: Remove the # in front of...
#32416
Read hondagx35's instructions, BUT change "device_tree_overlay=hifiberry-dacplus" instead of "#device_tree_overlay=hifiberry-digi"...
#28476
I could not find any instructions how to do this in the following section (but there is still a lot of good reading):
http://www.runeaudio.com/documentation/
Edit: I made the ASSUMPTION that the PiDAC+ is compatable with the Hifiberry DAC+, oops.