There are several ways to install wp themes, we do prefer using FTP but you can choose other ways.
Please read more about theme installation on the official website at http://codex.wordpress.org/Using_Themes
If you choose to use FTP, please do the following:
Step 1 : Please unzip the installable zip file that you downloaded from ThemeForest.
Step 2 : Upload theme folder to the theme directory of your WordPress via ftp.
Step 2 : Log into your WordPress admin dashboard and activate the theme in the Appearance => Themes section.