Question: If Jack and Katherine were a couple, how is it that Jack doesn't recognize Katherine's office when he goes there to confirm him and Tess to start working on the Trask deal?
Answer: Unless Katherine had recently moved to a new office, which it does not appear that she did, then Jack should know this was her office. However, it is also possible that he had just never been there before and they only saw each other outside of work. When Jack went to see Tess, he had a specific business reason for going there.
Answer: It is also stated in the movie that Katherine just transferred down from Boston, and she is staying at her parents house. She also mentions in another scene that her and Jack are now in the same city. Plus Tess helps plan her welcome party to get to know others in her department indicating that she is new there. Maybe Jack had not got a chance to visit her office yet, which would explain why he would not have known.