Instant web publishing has been removed for Server 13. WebDirect is the replacement.
The following assumes you have installed FileMaker Server properly and can get to the web site hosted by the FileMaker Server installation. If you are trying to host from FileMaker Pro directly this answer will not help you since web direct requires a server. You mentioned that you have FMS 13 on a machine so here is how to web direct from it.
Make sure WebDirect is enabled in the Admin Console.
Host your database on the server.
Open the database in FileMaker Pro and go to File>Sharing>Configure For FileMaker WebDirect... and enable the database for use via WebDirect.
Go to https://yourserver.domain.com/fmi/webd# in a browser and select your database from the list and log in.
Hope this helps.