Configuration File Method
Step 1: Login to your server control panel
Step 2: Click Configuration Files
Step 3: Open "7dtdsaves\serveradmin.xml" by click "Text Editor" to the right.
Step 4: Find the line that looks like this "<!-- <admin steamID="XX" permission_level="0" /> -->"
Step 5: Replace XX with your Steam GUID. You can use https://steamid.io/ to find your Steam ID.
Step 6: Once added, you will need to remove <!-- and -->" so that the line looks like this :
<admin steamID="xx" permission_level="0" />