To get Wakka's sigil you need to have all four overdrives, which you have. There is then a 50% chance that the Jupiter Sigil will be offered for coming First Place in a League. All you need to do is finish a League and exit to the field screen (the one you move about in). Save the game. Go into the blitzball menu and highlight League. If the Jupiter Sigil is offered then play the league. If not, sorft reset (all shoulder buttons + Start + Select), load the game and look again.