Mapping subdomain to amazon EC2 instance

I am using AWS OpsWorks and i have a domain managed in GoDaddy. Say the domain name is mydomain.com.

I am trying to map my EC2 instance to sub.mydomain.com. What i did is to edit the DNS Zone File and add an "A (Host)" with 'host'='sub' and 'points to' = . I waited for 48 hours but still, the sub domain does not point to the EC2 instance and the host does not exist.

Am i missing any steps?