I'm having trouble adding the Source button to CKEditor 4's toolbar. I just downloaded the new CKEditor today.
I'm using a config object named oConfig:
oConfig.toolbar = 'Custom';
oConfig.toolbar_Custom = [
['Bold', 'Source', 'Italic']
];
The toolbar shows up with only the Bold and Italic buttons. This example from CKEditor's docs tells me it should be working.
There are two reasons why it may be happening:
You have downloaded the basic package, where the sourcearea plugin is not included.
You are using CKEditor in inline mode. Source mode isn't available in inline mode yet.