I'm sorry to hear that you're having a problem connecting with Netflix.
You should be able to add another device to your Netflix account just by launching the Netflix app. It will prompt you to enter in a code on the www.Netflix.com/activate
website. I believe you can have up to six devices for each Netflix account, but that would be something that Netflix would be able to confirm.
I don't see any problems with the security certificate on the App Store, but I'll be happy to send that up so that we can look further into why this might have happened.