Hi,
Take a look at the sample file found in the “OptiSystem 12 Samples\Advanced modulation systems\PSK systems” folder named “DPSK Step 7 – Transmitter and receiver with eye diagram.osd”. You should be able to take the same approach to observe the eye diagram of a QPSK system.
Generally though since higher order modulation formats have more than 1 bit per symbol the eye diagram is less often used, because there are multiple eyes. Constellation diagrams are probably more useful for visualizing the signal and the error vector magnitude can be calculated from it.
Cheers!