it's pretty simple...
create the subdomain FIRST under domains >> manage domains >> add new domain/subdomain (domain to host would be moodle.millermc.net) make sure the php version works with moodle (i don't know if both or which one) go through the rest of the process and create your subdomain.
to add moodle...
go to the One Click Installs under Goodies
go to Install new website software - Advanced mode
select your new subdomain (if it's not there, try back in a few minutes)
then select an already created database to use, or scroll even further to create a new database
if new database, create your database name
Use Existing Hostname or at the bottom of that dropdown is create new hostname (this is the hostname for the DATABASE):
There is a delay while DNS propagates for new hostnames.
First User: select one already made, or select create a new one at the bottom of that dropdown
if new username, create New Username, New Password:
then select install if for me, if there are errors, they'll let you know