With the voltage/current source/sensor blocks in SimPowerSystems. You basically want to make sure your Simulink equations are of the form:
V = function(I) or I = function(V)
That way you can measure voltage, do the calculation, and apply a current back to the circuit -- or the opposite way if need be.
As a simple example, I'd point you to this page, which is the power_VariableInductor model. If you open up that model, right-click the "Variable Inductance" block and select "Mask > Look Under Mask" to see what I mean.
- Sebastian
