To install a PHP-Nuke theme you need to do the following steps:
First of all you should unpack the template package using the instructions from the "Unzipping Template Package" section.
There will be a folder named "xxxx" (a four digit number). This is the PHP-Nuke theme folder. Copy this folder to "/nuke/html/theme", where nuke is the folder to which PHP-Nuke is installed.
The next step is to select a theme from the list of themes: "Your Account > Themes".
If you want to use it as the default theme for your site, then move to "Administration > Preferences" and set the Default theme to the theme you bought. Notice that you must be a member of the administrators to change the default theme.
"Administration Panel" screenshot
"Change Default Theme" screenshot