You can find your License Key / Purchase Code in the email received upon the purchase. You can also log in to Your Account and navigate to the Licenses page to list your purchases.
If you've previously purchased LayerSlider on Envato Market, then you can also download your license certificate from CodeCanyon at any time. Just navigate to your Downloads page, then find LayerSlider and choose the License certificate & purchase code option from Download button. Your Purchase Code is inside the downloaded text file.