Need Help Saving iTunes Preferences
Generally, even when the actual song files are located on another drive like you state yours is, the iTunes library is still in the ~/Music/iTunes folder on your main computer drive. You can usually get there in Finder by clicking on Music in the sidebar.

Copy the above folder (iTunes) from your old computer to your new one, making sure to put it in the same place. If you already opened iTunes on the new computer, there will probably already be an iTunes folder there, and it's okay to drag it to the trash.

Once you've copied the folder over and you plug in your external hard drive, open iTunes while holding the Option key on the keyboard. When you see a message that asks you to Choose iTunes Library, click Choose Library. Navigate to the Music folder and select iTunes. At this point it may take a few minutes for iTunes to load, depending on the size of your library, but everything should be in place.