SSL certificate installation is closely connected to your hosting service provider. ST Registry (www.nic.st) provides only domain parking service which is limited with DNS zone management, domain redirects and email forwards. Once SSL certificate is issued and appeared in your account with status "ACTIVE" - you need to refer to your hosting provider for instructions concerning installation process or contact hosting provider support for assistance since mostly SSL Certificates are installed accordingly to your web server configuration. For example Java based web servers or Apache servers, also there are different applications that can be used for installation (WHM/cPanel, Oracle Wallet Manager etc.)
You can get necessary information such as CRT, private key within nic.st account. To get this, please open 'Manage Service' menu in 'My Services'. Then click on your SSL.