Without downloading the script and testing myself, it seems all you have to do is edit this part of the script
SCENE_MENU = { # XP Version
# index => [Menu Name, Menu Scene, Scene Args, Condition],
0 => ['Inventory' , Scene_Item , false, '$game_actors[1].hp >= 0'],
1 => ['Skill' , Scene_Skill, :actor],
2 => ['Equip' , Scene_Equip, :actor],
3 => ['Status' , Scene_Status, :actor],
4 => ['Save' , Scene_Save, false, '!$game_system.save_disabled'],
5 => ['End' , Scene_End ],
} if !VX
So say you wanted the Quests tab to be between Status and Save, then it'd be something like
SCENE_MENU = { # XP Version
# index => [Menu Name, Menu Scene, Scene Args, Condition],
0 => ['Inventory' , Scene_Item , false, '$game_actors[1].hp >= 0'],
1 => ['Skill' , Scene_Skill, :actor],
2 => ['Equip' , Scene_Equip, :actor],
3 => ['Status' , Scene_Status, :actor],
4 => ['Quests' , Scene_Quest, false],
5 => ['Save' , Scene_Save, false, '!$game_system.save_disabled'],
6 => ['End' , Scene_End ],
} if !VX
Replacing
Scene_Quest with whatever class name it's supposed to be based on your quest script. For example, if the script call to open your quests was
$scene = Scene_CustomQuestSystem.new
then it'd be
Scene_CustomQuestSystem.
Of course, sharing what quest system you're using would help me.