Hi Deena. Have you tried accessing the exam via the academy?
Mendix Academy - Advanced Developer Certification – Get certified should open up the exam.
I just received a reply from the Mendix Academy.
They have provided me with a new email which includes the actual button to start the test.
My advice for you will be to send over an email to firstname.lastname@example.org regarding this issue so you will also receive a new email to start your test.
I've had the same happen to me, which is a shame. The guide tells to have everything set up, make sure there won't be anyone disturbing you, make sure there's no interruption on your internet or electricty. Now it turns out I can't start the exam.