WordPress Toolkit is available to be installed in the WHM Marketplace interface.
Access the WHM Marketplace by logging in to your WHM as root and going to Server Configuration >> WHM Marketplace. On this page, access the "Add Extensions" button in the top right as seen in the screenshot below.
In this list of available Extensions, you will find "WordPress Toolkit" and the option to install this package as shown below.
Pressing the "Install" button will install the WordPress Toolkit on your server.
WordPress Toolkit can also be installed via the command line as explained here: How do I install WordPress Toolkit via the command line?