After extensive googling and searching, I cannot find out when and how to unlock deadbeat dungeon. I remember seeing a forum post somewhere saying that it unlocks in Ch2-3, But I'm already chapter 4 and do not have it. I'm normally pretty good about going through all the events I can before advancing chapters too...

Anyway, I want to grind up some of the characters I don't use frequently so I don't miss any events, and I heard Deadbeat Dungeon is the scummy way to do that. Could anyone tell me how to unlock it?

In the beginning of ch2, there will be an event with a deadbeat alchemist in Yuidora alleys, then the dungeon will appear on the map.
Initially it has only one level, new levels are unlocked every chapter or two.

Edit: Yeah, it's chapter 1.

Shoot, thanks for the headsup guys. Is it really that good at levelling up? I'm considering restarting just to unlock it, because grinding sucks

EDIT: Upon selecting new game, I discovered that I accidently left 2.0 maps off anyway. Restarting

As far as leveling goes - I leveled Lily from lvl1 to about lvl25 with a single visit to each of the 5 maps.
King Putetto is a good source of XP too.

With the King Putetto, you want to hit it but not kill it.
It will give 40 Exp per hit so the idea is to hit it then recover (if it hits you) and hit it again.
It has an AOE attack every 3 turns if I remember.

It's in the 5th Dungeon (Deadbeat God) which is base 70 so you probably won't have it open for a while.

There must be another trigger.
I triggered it at Chapter 1 in my save but all my chars are level 99 and I have a complete inventory.

It may be like worldendDominator wrote about chapter 2, something you create in the inventory is also required to enable it?
I guess make sure your characters are up a few levels and see what happens at chapter 2.

When you installed, did you add the 3 patches?
Append 01
Append 02
Version 2.00

Restarted and reinstalled about 3 times still cant get this deadbeat dungeon ill try once more 2night then ill probably rage and give up

Get a town event in chapter 1.