You can use the application page on Software Informer to obtain a link to the download or you can use its official website. If you still have the registration details, you will be prompted to enter them as some point during the installation procedure.
Your serial key/registration data might not work with all versions of the Graphics Suite. Latest version is X6. If you have purchased this version, you can use the details to register the application. For any other help, you might need to use their support options.