<Administrators>
<unsignedLong>Your steamID64 here</unsignedLong>
</Administrators>
So if your steamid64 is 76561197980321925 you should set it as follows:
<Administrators>
<unsignedLong>76561197980321925</unsignedLong>
</Administrators>
<Administrators>
<unsignedLong>76561197980321925</unsignedLong>
<unsignedLong>12345678910111213</unsignedLong>
<unsignedLong>13121110987654321</unsignedLong>
</Administrators>
Save and restart server. In-game you can now open the admin menu with alt + F10 key combo.