I see that FTP Advanced is disabled in my certificate.
How can I enable that? Can I upgrade the certificate?
Do you mean you upgraded to Opus 13 and the FTP feature disappeared? You should still have it. Make sure you're actually installed the new certificate and aren't just running off the stock 30 day certificate.
No, I don't think I ever had FTP Advanced. I was just thinking about including it with Directory Opus 13. I have already bought the new 5 license certificate. Can I still upgrade it to include FTP Advanced?
PS: I'm having problems with your forum. Everything is taking a long time.
It's been down for a few minutes for maintenance a couple of times today, but should be OK now.
Are you saying that I have to email someone?
No, Login on "My Account" with your Email Address and Registration Code.
Thanks. 
It doesn't appear that they have PayPal there for doing that.
Unfortunately this payment could not be authorized.
Could not contact the Payment Server.
Happened to me too, however now it works!
It's working for me too now.
Something that I noticed when doing it is, it said my forum account is unlinked.
So, I linked my new code in my forum profile settings, and it now says that it's linked.
Yup it said unlinked for me too, that was because the updated registration code for Opus 13 counts as a separate thing, so I had to manually link Opus 13.
In a way, the Opus 12 registration code becomes "orphaned" once you upgrade your existing license to Opus 13 since the registration code associated with your e-mail address in the GPSoft Self-Service system has changed, that's why it said "unlinked" on the Self-Service page (but on the forum it was still Opus 12).
So, does that mean that everyone who upgrades to Directory Opus 13 have to manually link their new code like I did?
I think in order for "Opus 12" to change to "Opus 13" for already-linked forum members they need to re-link manually (but it's only a single click in the forum profile).
UPDATE: it happens automatically (but with a delay):
This is just for the GP Software people. It's only minor. As I have already said, there is no PayPal.
But when you click on "What's This?" on your credit card details, you get a 404 error.
The forum runs a scheduled task to do that automatically, but it doesn't happen instantly - relinking can be done manually as you've discovered if you don't want to wait.
That should be fixed now, with it happening automatically again (as long as you upgraded the licence that was linked previously).
The automatic update is only done every 6 hours, but if anyone wants to refresh their own status without waiting for that, go to https://resource.dopus.com/my/link-opus and click the "Load Details" button.