iMessage "Not Delivered" on my Macbook Pro Mid 2012
What you are trying to do is send SMS messages from Messages on your Mac to people without iOS devices, correct?  (Just fixed this for my wife this morning.)

Be sure to add your Mac to your iPhone's Text Message Forwarding.

On your iPhone:


Go to Settings
Messages then select Text Message Forwarding
There you will see a list of devices that use your Apple ID for Messages
Add your MacBook if listed.
Enter the code displayed on your Mac into the iPhone dialog when prompted.


Now you should be able to text from your Mac using Messages.

Apple support documentation for further reading.