In case you wish install the theme via FTP, proceed as follows:
- Unzip theme-name-main-files.zip file you get from themeforest.net
- Now unzip theme-name.zip file located into theme-name-main-files folder
- Login to your hosting space via an FTP Client
- Upload the extracted theme-name theme folder into wp-content > themes folder and wait the upload.
- Activate the newly installed theme by going to Appearance > Themes and clicking the Activate button.