First, build and run the EchoServer.
You can set it to run on a specific port, or allow to pick an available port.
Now start the echo server.  It will output the port it started on.

You can now use telnet to connect to the echo server and test it.
Using the Terminal (/Applications/Utilities/Terminal.app)
type in the following:
telnet localhost [port]

where "[port]" is replaced by the port the Echo server is running on.

Type in anything you want, and hit return.
Whatever you typed in will be displayed in the Echo Server, and echoed back to you.

To end your telnet session hit Ctrl-], and type "quit".