Installing Coda Internal
Go to http://www.codamod.com/internal and grab the latest copy. Remember to launch the internal version through the "Coda Mod Internal" shortcuts and not the "Coda Mod Beta" shortcuts. Once you install the internal version you may use the internal version's auto-updater to get newer internal releases. You can check if you are playing the internal version of Coda at the Main Menu. Internal releases should have "Coda INTERNAL x.x.x" in the lower left corner of the main menu.
You should always run the auto-updater before playing an internal release. We usually release daily builds for our play testers. The Internal build will be a separate installation from the official releases of Coda. This means you can have both an official release and an internal release installed alongside each other. The two installations will not affect one another. The auto-updater for the internal build will update you to the newest internal release. The auto-updater for the official releases will only update you to the latest official release. The official release's auto-updater will not update it's installation to an internal release and vice versa.