The Hosts tab of the editor allows you to define and modify the set of host computers that will participate in your experiment. When empty it looks like this:
Define a new host by typing it's hostname or IP address in the empty Host Name box in the empty row at the end of the table. Hitting the enter key commits the change and assigns a unique host id to the hostname. This host id is what is actually used to bind rings and programs to that host.
Change an existing host name by simply typing a new hostname in a filled in box and again, hit enter to commit the change. Since host bindings are represented by the host id, changing a host name effectively moves all the rings and programs that had been associated with the old host to the new host.
Finally to remove a host from the list of hosts right click in the table line containing that host and select Delete from the resulting pop up menu. If a host is referenced by a ring or a program those references must first be deleted before the editor will allow you to delete a host from the table.
Here's an example of a filled in host table: