In the mobile, there's a gift icon over to the right of your camp screen.
PLEASE NOTE: in order to be eligible for the Event (see the icon), you MUST have the 1st quest in Isis complete & be Level 35+. You have until Monday night (before the Tuesday update) to meet those requirements & be able to claim the Superheroes Event. Otherwise, the offer expires.
PS: Because Pixel had to delay until Wednesday to make the event available, I'm not certain whether there's be an extra day to claim it...