Here's a link to a Google Map showing how to get to Duke.
I'll be playing there tonight, as it happens.
Quick directions - go to Bakurocho station on the JR lines. Also Bakurocho-yama station on the Toei Shinjuku subway line and Higashi-Nihonbashi station on the Toei Asakusa line all connect at that station.
Go to exit A2. Exit and turn right down that side street. Pass a Daily Yamazaki convenience store on the left side, then the next building on the left will have a stairway down to the B1 floor with signs up for Duke and the other restaurants in the building. Down the stairs, turn left, the door to Duke is right in front of you.