How to enable echo service for tcp or udp on well-known port 7 on Mac OS X 10.8 or above?
AFAIK the echo service isn't implemented in OS X. You may use either a Java echo client/server or CocoaEcho instead.

After unzipping the file check the folder .../build/Debug. I tested CocoaEcho server/client in 10.9.5 and it still works though it was built for 10.7. If you use a newer OS X version you may have to recompile it.