Before proceeding with the skin installation make sure your site is on DotNetNuke 5.1.0 or later which is required. The skin also depends on the excellent DDR Menu which is included in the package and has to be installed before the skin!
The skin package can be installed like every other extension on a DNN 5 portal. You just browse Admin > Extensions or Host > Extensions and fire up the installation wizard. Just keep in mind that if you install it from Admin > Extensions, the skins and containers will be available only to the portal you're on. If you want to make the skin available to all child portals under your DNN installation then install using the Host > Extensions installer.
The only step during the installation that may appear strange is when DNN will throw an alert with the following message: "The package contains files with invalid File Extensions ()". Just check "Ignore File Restrictions" and click "Next".