It's outside of the worldmap on which you can move around. You can't walk there.
You can only visit it as part of the adventures of Trey, Brad, and Ryen.
What you have to do depends on how far you're with there events, but there is a step by step guide in the wiki:
Ryoko: https://renryuu.miraheze.org/wiki/Ryoko#Character_events_sequence
Elly: https://renryuu.miraheze.org/wiki/Elly#Character_events_sequence