By default, the simulation will be running when you create a new circuit. The status of the simulation is displayed in the lower right of a circuit view, and can be changed via the menu. Firstly - a little explanation on how the Simulator works. This should allow you to make the most out of it.
When a circuit is created or modified, the affected areas are partitioned up into groups of pins and connections that can be considered independent. Each group is then simulated as a separate entity (although still interacting via the components), with the simulation provided dependent on the group's complexity. Complex groups, such as those involving nonlinear components like LEDs, are slow to simulate. Groups that contain only logic pins, of which only one controls the value on those pins, are the fastest to simulate.
The results of the simulation are provided through several graphical means.
The pins on the components will display voltage sidebars. These are colored orange for positive voltage, and blue for negative voltage. Their length depends on the voltage level, and their width on the amount of current flowing through the pin. These can be turned off in the General page of the Configuration dialog.
Hovering the mouse over a pin or connection will display a small tooltip showing the voltage and current at that point in the circuit. Several components also provide graphical feedback - for example, LEDs and voltmeters or ammeters.
Lastly, there is the oscilloscope, discussed in the next section.