To access the DirectAdmin control panel, go to: https://(servername).hmdnsgroup.com:2222 where (servername) is the server the site is on. It is also possile to go to https://domain.com:2222 but be aware this will generate browser warnings about a name mismatch between the request (domain.com) and the certficate (which is tied to the server name). It is fine to continue if the certificate presented is from *.hmdnsgorup.com.
Login to the control panel is also possible by logging in to the client area, choosing the active service, and then selecting the login to the control panel from there.