You need to log in to your ThemeForest account and go to your http://themeforest.net/downloads page. Locate the theme that you purchased in your “Downloads” list and click on the “Download” button. You will see a drop-down appears that will contain License certificate & purchase code in last position. You need to click on that and download license file with txt extension.
After you have downloaded the certificate you can open it in a text editor such as Notepad and copy the Item Purchase Code. Finally, use this Item Purchase Code when registering on our Support Site.