How to activate the child theme ?
The child theme comes with the full package that you can download from ThemeForest.
Make sure that you previously uploaded the main theme before installing the child theme.
Step-by-step guide:
1. Go to your download tab in ThemeForest (http://themeforest.net/downloads) and download “All files & Documentations”. See screenshot: http://d.pr/i/12oHu
2. Unzip the file on your computer
3. Go to the /Theme/ folder and you will see the child theme zip file (eg: lush-child.zip) which is the child theme that you will upload to your Wordpress
4. Go to WP-Admin > Appearance > Themes and click on 'add new'.
5. Select the child theme's zip file and upload it.
6. Activate it.