Don't read this unless you want it spoiled for you.
His hideout isn't in Newgate prison. It's in big ben in the museum. To get there, you have to complete the rest of marleybone's main quests. It's a tower dungeon that takes about 2 hours alone.
Just to give you a sense of where you are.....if you've already defeated Katz and talked to Grubb and built the clockwork, he tells you that Meowiarty is in Big Ben. However once you get to the museum - I think you're told to talk to Sherlock in the Museum - you find out you have to do Counterweights East and West. THEN you get to work your way up Big Ben and finally face Meow Boy. If Baxter hasn't told you about unlocking all those doors yet, you have some things to do before you face Meowiarty.
I have done every quest in mb to where the only 2 i have left are the cats and find hideout in unknown... yet for some reason it wont let me go into big ben yet what do i do
I have done every quest in mb to where the only 2 i have left are the cats and find hideout in unknown... yet for some reason it wont let me go into big ben yet what do i do
If your quest is "Stop That Cat", and you are on the part that says "locate hideout", you need to go to Gibson O'Leary. This map shows where he is in Newgate Prison:
Goodness gracious. Open your quest log file, and find the quest "Stop That Cat". Click on the Information/Dialogue Bubble in the upper left corner of the quest box. It will show you what you have done already, and what you need to do "after that".