To join an event you must be a member of the group the event was created in. Events created in groups you are not a member of will not be visible to you.
You can join an event from within the Activity feed by clicking on 'Join Event' button:
Another way you can join an event is by visiting the group the event was created in and clicking on the 'Events' tab. Here is where you will see a list of all upcoming events. Simply click on 'Join Event' button for the events you are interested in.