I know everyone will spam forum with post about how to get adult and master class and stuff like that so le me try get that in this post so later when patch come we don't have problems with it.
Here in this post you will find everything you need about it including map where to farm Korin item.
Map where you can farm Korin item for quest :
Adult and Master Class Quest Guide
Step 1: Once you reach level 30, find and destroy the crate pictured below that you'll found at various locations in the Korin Forest zone. If a create isn't at one of the locations it's because someone probably destroyed it recently. You'll either need to wait until the crate re-spawns, change to a different channel, or try and find one at a different location.
You can do this step, the next one, and a few others before you reach level 30, but you'll have to wait until level 30 to complete the quest line. If, when you begin to focus on becoming an adult, you notice that some of the quests aren't there, it's probably because you've done them already at an earlier level.
Step 2: After destroying the crate, loot the following item, right click on it, and accept the quest.
Step 3: The quest will take you to this NPC. Turn in the quest and accept the follow up. Don't worry about completing the follow up quest just yet and don't worry about no "X" showing up on your map for this quest.
Step 4: Just across from the NPC in step 3 (not standing right next to him) there should be another NPC quest giver with only one quest. Accept his quest.
Step 5: The quest you get from step 4 should take you to the girl pictured below. Turn in the quest and accept the follow up which should have you kill 10 enemy boars just outside of the small town. The boars you need to kill are the ones with the staves and not with the swords.
Step 6: After you complete that quest. Turn it in and receive the follow up which should take you to a man at the top of a giant hill.
Step 7: Turn in the quest and accept the follow up. The next quest will take you to the base of Korin's tower where you'll meet a girl. Turn in the quest there and talk to her again. She should now teleport you to to the top of Korin's Tower.
Step 8: Turn in the quest that has you talk to Korin and accept the quest he gives you. You should now have an item that looks like a bit like a tea kettle in your inventory.
Step 9: When you're ready, right click on the item which will summon four transparent Korins (two at a time). You must defeat all four to complete the quest. If you're having trouble defeating them you can stand where I am in the image below and fight them one at a time instead of two at a time.
Step 10: Turn in the quest and you will transform into an adult as well as receive your first HTB skill.
Step 11: Now, to choose a master class, talk to Korin again (choose the 3rd option). He will then present you with two items, each of which represent your master class choices. You will be permanently stuck with whichever Master Class you choose. There is currently no way to change your Master Class after you've chosen..
Click to view which master class each weapon represents
Step 12: After you choose your weapon Karin will give it to you. Right click on it and accept the quest it gives you.
If you're a Human your quest will take you to the Majin zone. If you're a Namekian you quest will take you to the Human zone. If you're a Majin your quest will take you to the Namekian zone.
Once you get to where you need to go the quest giver will give you a unique quest with the reward being your master class weapon (although you won't be able to equip it yet). He will then taken give you another quest which will take you to a special arena where you will have to defeat a powerful enemy (each enemy is class specific).
After you've defeated your enemy turn in the quest and you will recieve your master class and can now equip the weapon you got earlier.
I did not make this guide my self, it is from this page : http://www.dbocom.com/forum/showthread.php?t=6339. I could just send you all to that link but I was bored so wanted to make guide here, so I did that.
Enjoy your game.