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.
1. Go to your download tab in ThemeForest (http://themeforest.net/downloads) and download “All files & Documentations”. See screenshot: Click here
2. Unzip the file on your computer
3. Go to the /Theme/ folder and you will see the child theme zip file (eg: alaska-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.