Expired domains are retained during the grace period for the previous registrant to renew. During that time, only the previous registrant has access to renew that name. To renew your domain, visit myCP -> Manage Domains -> [your domain] -> Domain Renew -> [your domain] -> Add to Cart and follow the steps.
If you have any issues or troubles renewing your domain submit a support ticket requesting to renew the domain and the hostmaster will manually renew the domain for you.