Before you will be able to post topics on forum, you will need to obtain Purchase Code for product you purchased. Please follow steps below.
1. Navigate to to Downloads section in your ThemeForest profile and find the theme you purchased.
2. Click on Download button and on sub-menu click on License certificate & Purchase code (image below).
3. After downloading the file, open it and find the line that starts with Item Purchase Code. Code after colon is Purchase code you are looking for (check image below).