You must have purchased (or have in your cart) either a General Admission Ticket or a Package that includes a General Admission in order to purchase add-on items. You should create an online account at checkout if you plan to come back later and purchase additional items. This enables the system to verify you meet the prerequisite above.
Ticket holders should expect a 3-day event. Photographs and autographs will PROBABLY NOT occur on the same day as your tour/chat/event. Regular tours are fit into the schedule where possible.
The schedule will LIKELY change! Be flexible!