(Stealth option) If you use the side door to the club you will be at the bottom of a flight of stairs, you can stealth up the stairs and across to Johnnys office.
(Conversation option) Speak to Ricky sitting at a table in the club, with a high enough combination of Persuasion, Intimidation and Seduction he can be convinced to allow you upstairs. Speak to the guard at the bottom of the stairs and inform him you have permission to see Johnny.
(Combat option) Fight through the Tong to get upstairs.
Once in Johnnys office a conversation will start, you will be interrupted by a man on the TV.
He will tell Johnny to shoot you, then tell you to kill Johnny so that you can discuss Barabus.
Kill Johnny to complete the quest.
Update: Upon receiving quest:
You need to get to Johnny in his office upstairs at the Glaze nightclub to get information on the whereabouts of Barabus. The password to enter Glaze is "725" and you better do this without a big commotion.
Update: After speaking to Johnny and the Mandarin:
Kill Johnny and the Mandarin will discuss Barabus' whereabouts with you.