Normally, EnergyPlus does not track air pressure differences between zones, outdoor air, and HVAC system nodes. To enable EnergyPlus to be able to track air pressure values, you will need to add a series of AirflowNetwork objects to incorporate CONTAM calculation processes (read here for more info).

Just so that you know, AirflowNetwork objects require lots of detailed inputs. Luckily, someone has already asked a similar question for modeling ventilation between zones with external pressure, so there is lots of overlap there for your question regarding pressure differences between zones and outdoor air.

To examine output results from EnergyPlus, you should run a simulation and then open the report data dictionary (RDD) output file. This will contain a full list of every output variable that EnergyPlus can generate in a CSV output file from your IDF input file. Find the Output:Variable object you are interested in (perhaps AFN Linkage Node 1 to Node 2 Pressure Difference or AFN Node Total Pressure), copy that entire line for the object from the RDD file and paste it into your IDF input file. Run a simulation again, and then you should be able to see that output variable in the CSV output file.

I would highly recommend referring to the EnergyPlus example files that showcase the AirflowNetwork objects (e.g., AirflowNetwork_MultiZone_House.idf).