This musollah is not open to public but if you can talk to a staff there, they just might give you the access. I was attending an event at NLB, and needed to perform my solat before going to my next appointment. So when I asked someone there, a kind soul brought me to the musollah at B2 level.
I just want to say kudos to NLB management for preparing a spacious solat room for their staff! A praying staff is a happy staff.