Go to a web page on the server machine and put the following URL in: http://127.0.0.1:16000 That is the local machine's IP to see itself (you can substitute a LAN or public IP too) and port 16000 is the FileMaker listening port for Admin Console.
Open the browser and go to:
http://[server name or ip]:16000
That will open the server admin page. From there, click on "Open Server Admin Console" and the tool will open.
If you're on the server itself, you can substitute "localhost" for [server name or ip].