Hey, I took the online GMAT exam twice and the profile completion status on my mba.com profile is 72%. It really doesn't matter.
On the Test Day:
-> Login to mba.com 15 mins before your scheduled exam time. In "my account" section, you can find your online GMAT appointment and a "launch exam" button (visible 5-10 mins before your scheduled time).
-> You are then redirected to the examity portal and connected to a proctor. The Proctor will guide you through the procedures. Also,Install the secure browser on your PC.
The process is pretty intuitive. So, I would recommend you not to worry about these basic procedures. Just be cool and do your best.
All the best!
Posted from my mobile device