Hellbound -> Help in tully's workshop lineage 2 monster codex

Hi all

Yesterday a group of friends and I went to hellbound to go to the tower of Beleth.
We crossed the base city and got the key to enter the tower.
Overcome the maze without problems and got to the npc Jerien (or something like this) and enter Tully's Workshop.

And here we get stuck us as we saw no door or anything to continue.
People Say That We Must find a "mob secret key" to continue, But How do we find it? What should we do there, we need some special skill or some item?

So we're stuck in the first instance in Tully's workshop without knowing what to do to move forward into the following instances
We killed all the mob that was in the room but nothing happened.

Please help, we must do to continue?

Thank you and sorry for my bad English. (Using google translator)