NOTE: Before attempting this, make sure your either you've have passed Fortree cityand on the way to Mossdeep city (It doesn't matter if you have done anything else you just have to be up to the part where you have passed Mossdeep city.

Firstly go to the last tree house in Fortree city, that has the guy next to it that says Sniff... You, uh...smell singed. Where you at a volcano or something? Next go to the Kid that has the Wingull next to him talk to him then talk to his Wingull (If the Wingull went when you talked to the boy its O.K.). It will fly off to Mossdeep city. When you fly/reach Mossdeep city, go to the house where the staris are to Mossdeep Space Center "The closest place to place". There is a guy and a girl in the house. Talk to the girl then talk to the Wingull (If the Wingull went when you talked to the girl its O.K.) Then fly back to Fortree City and talk to the boy. You will get the Mental Herb!