Hello, the stress concept does not exist in DEM simulation.
You only have forces and displacements. So, to measure something like a stress, you need to measure some forces and apply some calculations with these measured forces in order to get a kind of stress.
You can take a look in the "PlugIn_InitSensor.hpp" and the "PlugIn_InitSensor.cpp" files in the "2.0/Example/Continuous/TensileTest/". This test is related to a tensile test applied on a cubic sample. In this case, the stress is simply computed as "F/S" where "F" is the total measured force applied on the sample and "S" is the section of the sample.