Where is the post game on dragon quest 9?

Answer 1

Simply put you need to follow the main quest through to the 'end'. After you've beaten the 'final boss', you continue playing. New quests are open to you, any that require the game to be finished first are part of the 'post game' content.

Answer 2

You get to go to the tower of nod by defeating quest 39. You get that quest from the girl in port lafflan. You can also get a new vocation.