You can go to the passholders page and there is a place where you can put in your exchange certificate number.
Beware the site can be a bit fickle! I didn't have any trouble until last week, and when I put in my number, the login page kept coming up. I posted here and someone said to close Spysweeper and antivirus software so I did that and then I was able to get in. Then last night, I tried to get in and it said I had an invalid number! This morning I could get in, even though I hadn't closed Spysweeper and the antivirus software.
I have an exchange certificate too and I usually have no problem accessing the site with it. But, I have been having a problem with the site since Saturday, its saying invalid number too. I hope it works soon....
I have not been able to access AP site for a few years. I have written and spoke to them several times. They don't know why I can't access it either. I finally gave up! Hope you have better luck!