If that template is usable with Kentico it should have come with some instructions for setting it up and hence you really should refer to the seller. In most cases you would have a Sites Export Package that you should Import to your site.
However, you can have a look at https://docs.kentico.com/display/K82/Creating+web+templates which gives you the steps to add a template such that it shows up in the Site Creation Select Template step. The key thing here is you need to ensure you have a Sites Export Data Package.