After you’ve downloaded theme files via themeforest you can upload your theme using WordPress import theme screen.
In Appearance->Themes you can click to Add new and then on upload theme.
Next you can add your theme .zip file and click on “install now”.
After few moments (it depends on your internet connection and your server performance) the theme will be available in your WordPress installation.
After that you have to click on “activate” button under Tiam miniature.