To add the calendar to your sim.org calendar go to email.sim.org.
Then add webcal://simnigeria.org/public_calendar/sim_events.ics?id=0 in the 'subscribe via internet' screen

To add the calendar to your gmail calendar go to calendar.google.com.
Then paste this url in the agenda-url field webcal://simnigeria.org/public_calendar/sim_events.ics?id=0.

To add the calendar to outlook click on 'open agenda'->'from the internet'.
Then paste this url in the agenda field webcal://simnigeria.org/public_calendar/sim_events.ics?id=0.