Unfortunately, you fell for a well known scam. Apple just manufactures computers and has designed an operating system. Apple has no idea what's in your computer, doesn't monitor your computer, and never reaches out like this to gain access to your computer.
These scammers are just phishing for personal identifying information, your credit card number and authorization codes, for remote access to your computer and for the money they will be able to steal.
You didn't have to worry about your computer containing malware before. But you have given scammers remote access, and now you really have to concern yourself with malware and identity theft.
When you give scammers like this remote access, they get your they can plant spyware in your computer which ultimately allows them to hack into your passwords, emails and accounts and rob you blind.
You need to call your credit card company ASAP and tell them that your card is compromised. Make sure there are no unauthorized charges on it. See if you can get a charge back for your payment to Macroworld. Then cancel your card and get your card carrier to send you a new one.
Then you want to get your computer over to a real live local computer tech and have him debug it. Choose one that's in your community and that you know has a good reputation. Once it's debugged, you can change all your passwords and PIN numbers.
If you would rather not bring your computer to a local shop and you are at all tech-savvy, you could post a question to our verified computer techs in our Computer section here on JA instead, asking them to walk you through how to debug your computer, because there may be spyware in there. They can do that for you.
JustAnswer uses an independent third party background check service to verify the credentials of all of their experts before they are allowed to answer questions here. So it's safe to do that. Otherwise, you do not want to give any unknown company remote access -- ever -- unless/until you vet that company first.
Once your computer is clean you should also report the fraud to the police, the FBI at IC3.gov and the Federal Trade Commission at FTC.gov