First and foremost, please know that it is not necessary to know your computer ID if you use Toontrack’s Product Manager authorization system. The Product Manager (and a video explaining it) can be found here: http://www.toontrack.com/product-manager/
The Toontrack Product Manager greatly simplifies the installation and authorization process by taking out all of the guesswork. So, if you are looking for your computer ID and your computer is connected to the internet, please stop what you are doing and instead download, install and launch the Toontrack Product Manager.
If your computer is not connected to the internet, you will need the computer ID. It is found by launching the program and then clicking the OFFLINE authorization button.
The computer ID is based on various components in your computer hardware and can not be changed by the user. It CAN NOT be found in your system settings (Control Panel / About this Mac). It will only show after you have installed and then launched your product in your host.