I haven't received a confirmation email
First and foremost, please check your Spam folder. If you're a Gmail user make sure to check your Social and Promotions tabs as well. Messages will usually arrive instantly, but if nothing has arrived in your inbox within 30 minutes then it's time to do something about it.
This issue will usually affect email addresses with custom domain names e.g.:
You can submit a ticket to our support team who can activate your account manually, but it may take some time so the quickest way is to sign up using another email address.