Thank you for the post.
For upgrading or moving an existing installation consult Chapter 5 of the FileMaker Server 12 Getting Started Guide:
Which installation media was used when installing FileMaker Server 12?
If not attempted prior, then install using the full FileMaker Server 12.0v4 installer located in FileMaker's Knowledge Base article 11601:
The FileMaker Server Admin Console is a Java applet that may be accessed through a web browser. From the physical server or when remoted into the server, open a browser and in the URL type:
The same can be achieved from a remote machine by replacing “localhost” with the IP address of the FileMaker Server.
This will access the FileMaker Start Page and clicking on “Start Admin Console” will begin a download of the Java applet. Once downloaded, double-click the applet and login using the user name and password for the server administrator.
If the password is lost or forgotten, see Knowledge Base article 6989 for instructions on how to reset:
Java will need to be installed prior and if prompted, allow Java permission to run. The applet may be placed on the desktop for easy reference.
Updating to 12v4 did the trick.
The problem I had before was that the Java applet wasn't firing (there was an exception with Java trying to access certain ports). Access from localhost:16000 would bring up a blank screen and running the FMS12 start icon would say that the application wouldn't start.