To become an administrator in Space Engineers, you will need to know a couple things. First off you'll need to get your Steam ID64. To get this, open up steam and go to your steam community profile. Copy your URL to your profile, and go to the website steamid.io. There, you'll be able to paste in your URL to show you all the information you'll need. You're looking for the Steam ID64.
Once you have your Steam 64, keep this number available. Go to your TCAdmin user panel, and login with the TCAdmin login information which was sent to you in your purchase email. From there, click stop to stop your server. This is a very important step. Next, click on configuration files. Next to the "SpaceEngineers-Dedicated.cfg" file you will see a link "[Text Editor]" Click that. Within the text editor look for the XML Element
<Administrators /> You'll want to replace this with the text below:
Once you've added this code, save your config file, then go back to your TCAdmin user panel. Click on "Start" to reboot your server. Verify your server generates by watching your server's console within your web console. Once your server is back online, you should have admin authentication on your server.