If you haven't already done that, download Navio2 drivers and examples code here.
Move to the folder with the source code, compile and run the example:
cd C++/Examples/Barometer make ./Barometer
Move to the folder with the code and run the example:
cd Python python Barometer.py
You should see pressure and temperature values. Keep in mind that board tends to get hot from Raspberry and would show slightly higher temperature than it is in your room.
Temperature(C): 34.3509172821 Pressure(millibar): 1030.4646104 Temperature(C): 34.2971904755 Pressure(millibar): 1030.4639519 Temperature(C): 34.2795449066 Pressure(millibar): 1030.45554448 Temperature(C): 34.3018652344 Pressure(millibar): 1030.46921925
For further information see source code, which is mostly self-explanatory. As it takes some time for the sensor to perform conversion delays are added between calls to methods that start the conversion and read the resulting data. After reading both pressure and temperature values temperature compensation can be performed to get resulting pressure value.
More information about the chip is available in the MS5611 datasheet.