Use the various search fields to locate your event. When you see it, click the "Add Me" button in the right column.
You can also add yourself to the event from that event's details page. Under the "Event Actions" section, you will see an "Add Me" button. Click it and you will be added to the event.
This feature should be used for games in which you have been already assigned. It is not meant for you to say you are open to work an event.