To create an activity, you have to open the calendar and click on the 'Add+' link at the top. If it's a river outing, indicate the section and date in the title.
Video: http://www.screencast.com/t/ypMfOTZx4ab
To modify your activity, you must first open it in the calendar, then open it in full mode (click on the title or the 'Plus' link at the bottom of the window). Once the activity page is open, you must click on the 'Modify' tab.