OK, still using your original HD in the external case you should be able to boot up your system. Now go into the '''System Preferences''' The fourth grouping you should see a drive Icon labeled '''Startup Disk'''. Open it up there you will see your two drives click on the now internal drive (blessing it). Restarting the system should then switch the startup to the new drive (don't forget to pull your external drive when you hear the Bong).
OK, you don't see the new drive! Now what? If that is the case the OS files are not properly installed. You'll need to download the OS installer from the Apple Store using your external drive. Once you down load it, run the installer just make sure you don't format your drive! It will walk you though the install process and should bless the drive as well.