Copy the entire iTunes folder which is inside your home directory/Music of the old Mac to the external drive. Paste it into the same folder on the new Mac. Launch iTunes with the option key held down and point it at the same folder. Done. Alternatively backup the entire library to CD/DVD with the Backup to Disc option in the File menu, and insert the burned discs into the new computer. This second method is assuming you haven't already done the hard drive migration method.