-603 means that the user account is invalid. Please make sure you signed in Exchange with correct account.
Thank you for the reply. I am signed into Exchange with the correct account. I can download other add-ons from the exchange and they install correctly. The only problem I'm having is with the Portraiture plugin...
I emailed Imagenomic and they responded by doing a remote session with me to resolve the installation issues. Their customer service is awesome!
I can install it by following How to install the plugins in Photoshop CC 2014 - Powered by Kayako eSupport Help Desk Software, and make sure choose the Plug-ins folder by double clicking it on the choosing host screen. The path should be /Applications/Adobe Photoshop CC 2014/Plug-ins.
For using adobe exchange, you can try the new way to install add-ons by following: Another way to install Add-ons.