Accessible Calendar requires you to be logged in to a Google account, as Google calendar does not work otherwise. You can log in by clicking Login and Info on the first page (below the Calendar button).
For these buttons to work, there needs to be a day selected (highlighted) and an event scheduled for that day. If no day is selected, click Next Day to select the first day that is presented on the screen.
Make sure it is selected and click View Event. After that click Edit Event.
The reason can be that you do not have permissions to edit the event.
Make sure there is a selected day (a day that is highlighted) before clicking this button. When creating the event, you will be able to change the date, but a date is needed to start the event creation process.