There's one Citibank on Koshu Kaido down the hill from the South Entrance of JR Shinjuku (when leaving the station turn right and go down the hill). You'll see a stoplight and a "First Kitchen" fast food restaurant with a clock on it. Cross the street and walk for about a minute. It will be on your right. Can't miss it.

And if you're hungry, there's a ramen shop directly across the alley from the Citibank. On the right side entrance, there's a ticket machine and an english menu either on the top or on the side of the ticket machine. Take some of your newly gotten cash and get yourself something to eat...