You will have to create the DP-QPSK Transmitter by yourself or copy one from the sample projects, because you need to compare the electrical M-ary signal. The output of the DSP is not in a NRZ format.
First try with a simple back-to-back system without the fiber link and see if you can get that working, only later add in the transmission section. I am sure that you are aware, but the DSP already includes an Adaptive Equalizer if that interests you, it would make the project simpler at least.
Regards