Hi...
Supreme Kai: When you're in Gohan's D.U for at least the second
time, and after the battle with Goten head to
Kami's Lookout and face Picollo, after you have
beat him or time has run out and you won, you'll
appear above Babidi's Spaceship and ready to fight
Dabura, but ignore this fight and head to the
North-East part of the map, just North of Hercule's
City you'll find with Vegeta titled 'Mountains'
after your chat with Vegeta head to Babidi's Spaceship
and defeat Dabura, once again the red dot will appear
at Babidi's Spaceship, enter it, now Gohan will go
after Vegeta, but ignore your fight with him and head
to the West City, fly a little to the East and you'll
find some dark-green plains, near there should be a
scene with Supreme Kai titled 'Plains', after this
scene you'll unlock him.
Sighs...