You don't need to enable anything in calibre, once the Content Server is running it will be accessible over the Internet.
Port forwarding merely tells other computers how to connect to your
computer.
...
General guide to port forwarding, with descriptions for particular router models:
http://portforward.com/english/route...outerindex.htm
Once you've done that, you may want to get a dynamic DNS service like
DuckDNS, which will save you from having to worry about the IP address itself.