Hey Dan,
Welcome to the Forums and Welcome to GlowHost. We hope you will like it here.
Just to follow-up, after you have set the nameservers, you can then access each account's cPanel and Webmail at the following URLs:
domain.com/cpanel
domain.com/webmail
Where domain.com is the actual domain you are hosting. In fact, as a reseller, you can access all of your customer cPanel or Webmail (and your WHM) at any domain in your reseller account by using a forward slash + cpanel, forward slash + webmail or forward slash + whm
E.g.
random-domain.com/cpanel could be user to access domain.com/cpanel
random-domain.com/webmail could be used to access domain.com/webmail
domain.com/whm could be used for your-reseller.com/whm or random-domain.com/whm
/cpanel /whm and /webmail work on all of your domains and are simply redirects to the control panel services.
Also, if you look in your instruction letter, you can replace your domain with your customer's domains. and there should be an IP which will also work for all of your accounts. Finally, there should also be a hostname like nickname.domain.com which can also be used for all accounts. The IP and hostname methods can be used to access cPanel (or webmail or WHM) before you point your nameservers.
Finally, if you login to your cPanel, with your main reseller username and reseller password, you will see a drop down box at the top of cPanel, this will allow you to toggle between your cPanel and your customer's cPanels.
PS I am not sure what AMC is but if you mean 'Account Manager' or myGlow this is the place on OUR site where you pay bills, open tickets, and related things. You might find a link to YOUR main reseller account's cPanel in myGlow from which you can then change to other cPanels using the dropdown box described above, once you've landed in your main cPanel.
Keep in mind that at this time, the password listed for your main reseller account inside of myGlow may not be correct if you have changed it from the default password which we set the account up with (the same password which is in your welcome email) using WHM. If that is true, just update your password which is stored in myGlow to match the one that you changed it to in WHM and then the auto-login will work, if you wish to have auto-login to your main account, from within myGlow.