The Penta can be used either as a mono or a stereo compressor. Both the instrument and mic inputs route signal through only the left channel of the Penta, therefore the signal you should route to the receiving device when using these inputs will be from the left output.
You can use either of the line inputs on the rear to compress a mono line level signal. If compressing a mix of a track then you will use the Penta as a stereo compressor by connecting a stereo output of your mixer to the left and right line level inputs. Note that the Penta is not a dual mono compressor - the gain reducing action of the left and right channels is linked therefore the Penta is not suitable for compressing two signals from different sources (e.g. voice and guitar) simultaneously.