To single step through a simulation you first need to halt the running simulation. Click the halt simulation button on the toolbar, select the SimulationàHalt menu option or press the F5 shortcut key to halt the simulation. Note that halting the simulation is different to ending the simulation - ending the simulation returns Virtual Breadboard to design mode and hides any runtime tools associated with the components.
With the simulation halted click the source button and the Special Function Registers button from the PIC16c54 debug toolbar to open the source code for the PIC. The complete documentation to the debugging tools available to components of the PICMICROS package are available in the PICMICROS package specification and are only covered here briefly as an example of component runtime tool usage.
By having the Source code and the Special Function Registers open as you single step you can see what is going on inside the PIC and how it correlates to the rest of the circuit. Single stepping is an invaluable debugging tool and I am sure you will find the ability to single step plus change values in other parts of the simulated circuit to be extremely powerful tools in the development of your microcontroller applications.