Once you have setup you Digital Channels, click Tools -> Math Channels
Next, click Create, followed by Next
In the Math Channel wizard, Click Advanced, followed by the 'duty' button.
Place the cursor in the brackets in the equation editor and then from the drop-down next to the button for channel B, select the digital channel (in this case D0).
Follow through the steps in the Math Channel wizard, and then ensure that the new Math Channel has a tick next to it:
This will then display a waveform in the analogue waveform section of the PicoScope 6 display representing the duty cycle value.