Be careful with this. When I tried to do this at O'hare the agent got pissy and said its only for people that don't live in the US and told me to do it in the office instead
That agent is dead wrong.
Per the CBP website that is linked in the relevant
Global Entry Enrollment on Arrival thread:
Enrollment on Arrival is a program operated by U.S. Customs and Border Protection (CBP) to allow Global Entry applicants who are conditionally approved to complete their interviews upon arrival into the United States. The Enrollment on Arrival program eliminates the need for a Global Entry applicant to schedule an interview at an enrollment center to complete the application process.
When landing in an international terminal follow the signage directing you to CBP officers who can complete your Global Entry interview during your admissibility inspection.
Perhaps you went to the wrong booth or at the wrong time or day. At Chicago O'Hare International Airport,
Enrollment on Arrival interviews are available only at Booth 58 from 8:15 a.m. to 3:30 p.m. Monday - Friday.