Monthly shading/solar access can be entered by clicking on the calendar icon in the System Array modal.
Instructions on how to access the array modal can be found here.
After clicking on the calendar icon, the Monthly Solar Access modal will open. Enter the monthly average in the Average Solar Resource field and hit APPLY. This will apply the monthly average solar access to all months. You can also edit individual months by entering the monthly solar access for a particular month.
After you have completed entering the solar access values, click SAVE to return to the Array modal.