In iTunesConnect, select the app which you want to add an IAP to.

Click on features and you will see this:

http://i.stack.imgur.com/gAqOh.png

Click the plus. You will then need to select which type of IAP you want to make.

Then you will need to fill out all of the information for your IAP.

http://i.stack.imgur.com/HXFv5.png

If you have any trouble you can consult the IAP Set Up Guide.