You have to use the Control menu to select a compatible monitoring mode.
It's covered in the Notes.txt file.
Using DSD+ with external signal sources
DSD+ can be used with external signal sources such as a properly discriminator-tapped receiver
or SDR software such as SDR#. To use DSD+ with these sources, use the menus in DSD+ to
select Passive Digital Monitor mode and then to select the audio input device that carries
the digital audio signal. When running in this mode, DSD+ can be used to monitor conventional
analog and supported digital signals; DSD+ will not directly support trunk tracking.
