how do you unlock supreme kai?

Adult Gohan's DU - After defeating Goten, Visit Bulma in West city then Videl in the Plains area A little NE of Goku’s house or south of grandpa Gohan’s. After you talk to Videl Fight her in another “Plains” area NW of West City. It will then jump to Gohan Talking about Piccolo and Krillin being turned to stone, ignore that for the time being and head the NE of the continent and use R1 to search for Vegeta near some mountains. Talk to him then head back to Babadi’s Spaceship and fight Dabura (or if you want head for the mountains Goku fought Vegeta at and find Videl again and get the “Videl’s Kiss” Capsule for Great Saiyaman). In the same spot as Dabura fight Majin Vegeta. After, head to central city (the city in the central of the map right next to a large lake) and hit R1 again to search for Supreme Kai. Talk to him and gain his capsule

Go through normally until Dabura then head over to the mountains on the very north east of the continent. Search the one's lower down and find Vegeta, Afterwards go fight Dabura then Vegeta then make your way to central city. Search a little bit west of the city to find Supreme Kai and his capsule

Gohan's Second DU Go through normally until about Dabura. Then head over to the bottom portion of the mountains on the very northeast near the ice caps. Search the bottom portion of the area to find Vegeta and ask him what's wrong. Fight Dabura as planned to get Fat Buu switched out with Majin Vegeta. After defeating that nightmare, Gohan will have to fly over to and face off against Kid buu. Before that however, head to Central City and search a bit west of the city to find your target, Supreme Kai and his capsule.