Click on a single event of the relevant Section (Beavers,Cubs,Scouts), then on the little gear icon that will show you: Categories|get as iCal| Print. Then get as iCal .
This will download an "ics" file on your device. Open it (usually in your Downloads folder) and your calendar may give you some security warnings that you will need to approve; import , if you are given the choice.
The existing events of that Section are automatically inserted into your own device's calendar. Note that there is no synchronization so you should periodically check for changes on the website calendar.