Yes, you can host a domain that you registered somewhere else. You'll need to set up nameservers with your registrar. The nameservers you need to change to are included in the email you received with your account information. You can also log in to your client area with us and go to Services > My Services and click view details next to the product.
Exactly where you enter these nameservers and how will depend on your domain registrar. If you need help, please contact them, as it will be somewhere in your account with them.
Once changed, it can take 24-48 hours to propagate. This means it could be 24-48 hours before your site is visible on the internet when visiting yourdomain.com. Until it propagates, you can still log into and manage your site with the link in your account information email.