Quite simple, go to the place where there are 4 panels in a column, then go to the top one, battle the trainer, then go to the panel which leads to Mr. Fuego.
It's pretty easy, but first finish the Elite Four. When you go to Floarama town, move to the right (not into valley window works, but the route before it) and then surf, keep moving right and there!...
go to the fuego windworks. use surf to travel north until u see land that was unreachable before. search for a factory on the island (easy to find) and there u go!
well you 1.go on the bridge when you first headed straight for eterna forest and head left (there will be alot of water type so either bring repels or thunder type)then when you can get onto land go...