On poptropica i don't know how to play astro knight island. when i go to my things it will say astro knights?

I can't play astro knights. when i go to my hot air ballon to travel it won't have astro knights as an island to play. But when i go to my things you know how they will let you change the island at the top so you can see what you have from each island well it will have astro knights a one of the choices. I am confused HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!…