If you enter the message in the Message to All Recipients section, it should show up in the EMAIL for both signers and CC recipients.  I think the reason the CC recipient doesn't see this message in SIGNING is that messages don't appear for recipients who have completed their jobs in the envelope.  If you're trying to get the message to show up in Signing for a CC recipient, I don't know of a way.  Maybe someone else here knows...

It should also be noted that if your CC recipients are in the final spot of the signing order with no other Signing recipients in that order with them, they will receive only a Completed email notification and NOT a CC email, which will NOT include the message to all Recipients. 

You can modify your brand to add the email message to the Completed email notification as well as any Private message that was added in the process, but then that Private message would go to all Recipients along with the Completed email notification. 

Additionally if you have a Private message or want the CC to see the email, you can add yourself in the final signing order as a "Needs to View" Recipient, then your CC recipients will get the normal CC email with the Private message and the email and then you'll get an Email which you'll have to click and open, but not do anything. Your envelope will then advance and complete and all recipients will get the Completed notification.

Hope that helps.