It is revealed to you in an unmissable event at a certain point in the main quest. So continue with the main quest, and you'll gain permanent access to it.
Go to Alltrades Alley and go downstairs. You will find a priest. Talk to him to start the Gladiator quest.
newid isle. leave alltrades abbey and head west. there should be some small mountains you will have to go around. head south once you get around the mountains and you should see some sparkling bits.
I'm going to assume you mean Alltrades Abbey in DQ IX. it can be found AFTER completing the Coffinwell story and after going back (SPOILER) to the observatory for the first time (END SPOILER). it's on newid isle, and it's just north of the blue tree you land on after Stella takes you back to the Protocterate
you need to talk to all of the agrivated people, talk to the guy in the back, find Yhe martial artist and talk to him, talk to the guy in the back(again), go to the tower of trades (east), BOW (new party trick) in front of the door, go to the top, find Jack of Alltrades, battle him (Master of Nu'un), get a fig,and go back to Alltades Abby. Now you can change vocation.
After you get the starlight express working, and then get sent back to Earth to find the figs, you will come down near a blue tree. On the map you will see a castle, just head towards that.
When he is missing from the Abbey, he can be found at the top of the Tower of Trades, just to the East of Alltrades Abbey. Once found, he will then appear in the abbey, just north of the entrance. You need the Party Trick 'bow' to enter the tower as there is no key hole but it is locked, you can get bow inside Alltrades Abbey. When you get bow, assign it to one of yours and bow in front of the door of the tower. Get up the tower, which is easy enough. Just ignore the chests and pots and go for them later. Then you'll go straight ahead, up the shiny path of light. Then you'll watch a little scene of Jack turning monster then you'll have to fight him. Then he'll get all sulky and head back. Then you can change your vocation anytime !!
You can't but you can make one @ dragon quest monsters.com
Talk to all of the agrivated people in alltrades abby, talk to the man in the back, find the martial artist and talf to him, talk to the man in the back (again), go to the tower of trades, bow in front of the door,and get to to top of thetower. You found him, but now you have to battle him (Master of Nu'un). After you battle him, he will return to Alltrades Abby, you will get a fig, you can change vocation, and they all live happily ever after.
gittinghampalace
U have to fight him in warlic's quest(dragon egg)
oaklore dragon defend oaklore quest
nythrea's first quest you'll find it on that one