As of Builder 5, all Builder functionality is included in each Builder theme. Older Builder themes would rely on Builder as the parent theme, and each theme would be a separate child theme.
Installation instructions are therefor not the same.
For Builder version 5 and up
Once you have purchased your Builder theme, you will find the theme download in your membership area (https://ithemes.com/member/panel/downloads.php). Select the theme to download, and save the Zip file in an easy to remember location.
Log in to the WordPress admin area of your site. From there, go to Appearance > Themes. Click Install Themes, then Upload. Select the Builder.zip and click Install Now.
Finally, activate your Builder theme.
During the theme activation process, you will be prompted to generate a custom child theme. This is highly recommended. If you do not use a custom child theme, any modifications you might make to the theme template files, or the stylesheet will be overwritten when you update your Builder theme.
You can see a step by step overview in these Builder tutorials.
Note: For Builder that use a pre Builder 5 childtheme and the plain Builder theme as the parent theme, only(!) the Builder parent theme should be updated.