Hi Aida! In the Modelica Standard Library, there is an component that will allow you to find the voltage between two connectors. http://reference.wolfram.com/system-modeler/libraries/Modelica/Modelica.Electrical.Analog.Sensors.VoltageSensor.html You can find it under Modelica.Electrical.Analog.Sensors.VoltageSensor. You'll just have to connect the two pins to different parts of your circuit and after simulating, you'll find the voltage difference by expanding the sensor component i SimulationCenter.

It is always easier to provide an answer if you include your model file as an attachment, just something to think about in the future (: Hope that helps and welcome to the community!