The port D on the Photodiode is a Physical Signal input port (note the triangle shaped port). This is a unidirectional input which is different than the bidirectional electrical connection ports (+ and - with round ports).
A Physical Signal must feed this port. One way is to use the PS Constant block. It is found in your library browser under Simscape > Foundation Library > Physical Signals > Sources.
Alternatively, you could use a Simulink signal (output from another portion of the model or from a block in the Library Browser under Simulink > Sources). To connect a Simulink signal, you must use a Simulink-PS converter. This block is found under Simscape > Utilities.
Examples of this can be found in a number of Simscape and SimElectronics demos. One example is the Solenoid demo. This can be found by opening your help browser and going to Simscape > Demos.
