Login in Theme Forest with your buyer account.
Go to Downloads
Go to your purchase and select License Certificate and Purchase Code
Copy the buyer code from the text file
You will need the license code to register for a support account in client support system – http://support.wpestate.org/
And the same license to activate the theme
IMPORTANT: You must do this only once after you update or install WP Rentals v 1.21.1 . This will not be required again for future updates, unless you decide to reset database and start over.
If the theme theme is not activated will cannot access theme options and install theme included plugins.