That's a crazy question to ask unless narrowing it down.
Are you looking for an example for a 2D MMO RPG? Is the client code in Objective-C or Swift? What type of "server" code is in place.... That type of stuff I'd need to know to try give you good advice to get started.
Cocos2D is an amazing 2D scene manager, it takes care of all of the drawing stuff on scene. Mostly you don't have to worry about how it's drawn or memory issues. Just set up your scene, place some buttons, labels, and sprites, and you've got familiar with what Cocos2D can do for you.
To make an MMO, you shouldn't be worried about the Cocos2D framework but rather your web backend.
Usually modern MMOs have most of the code work done on the server side (for performance and security reasons, it's also easier to push patches/changes of code out). The client (code on the device) sets up a connection and handles a lot of the "display" and input code.
I actually built an online backend to my own game on iOS called the "Calis Projects Garden". Most of it is done with PHP and MySQL and I use plain old "NSURL" stuff to send and receive data, but it's far from "Real time" (since most requests take at least a second to send out and then get back). It does have user profiles, dynamic quests and events, and that type of stuff.
If you want to read more about it, click this text. Old post, but gives you an idea.
But - that took me 2 years AFTER release of my game (and solely focusing on that project) to do.... It wasn't easy.
Use Cocos2D to set up a basic scene. Just a background and a button.
When the button is pressed, use a way to request a URL and see can you pull down information (look into "Objective-C and PHP/JSON" or something).
Once you can do this - then learn how to maybe open a stream and have some Node.js code or something on your own server to connect to. Build your game.
Check out this framework - it's amazing, I used it before. It's a game focused "backend service":