Here is the secret to attaching the arms! Wayfaur sent this to us last night when my kids couldn’t attach the arms., 
****Dear customer,

Please kindly try to use the seat base to attach the arm.

Kindly lift the base up, fix the arm, align the metal clips of arm, then slide the clips of base down to the clips of arm. (If use the arm to attach the base, the metal pieces may pop out)

We hope it could help, If you need further assistance, please don't hesitate to reach out!