Tutorial
iPhone
To add this calendar on your iphone navigate to the calendar you want and click on the (WebCal Link)
The calendar app will open and ask you to subscribe to this calendar. Click Subscribe. Done. To hide the calendar just click calendars and uncheck the Fetlife Events calendar.
Andorid
I dont know yet. I will let you know when I find someone with an android phone. If you know, please write a tutorial and use the contact button above!
Google Calendar
Just click the 
and it will ask you if you would like to subscribe to the calendar.
If you sync your google calendar to your phone and would like to have this calendar on your phone check out this information from google help: https://support.google.com/calendar/topic/13950?hl=en&ref_topic=1665163
iCalendar on Mac
Just click the (WebCal Link) it will open it up in iCalendar and ask you if wou wish to subscribe.
Other Calendars
Most other calendaring service offer the ability to sync using an ics file. Below every calendar is an (ICS Link). Just copy that url into the calendaring application of your choice.