The link I posted will enable you to set up a Certificate for your DA login.
DirectAdmin doesn't use Apache for it's own control panel, so of course those instructions won't help you set up a secure default site for Apache.
I believe I've posted a How-To on how to do that. You can use the same Certificate for that, depending on the license you get from your Certificate distributor.
If I haven't, I don't have time to do it today, but since DirectAdmin doesn't touch your default apache setup, and since DA does use it (generally at /var/www/html) you can generally follow the instructions supplied by your Certificate supplier.
We've also offered this install as a service for some time; our post can be found on the Third Party (Advertising) Forum.
Jeff