You enter your license under the ‘General’ tab, which is on the Settings page, as shown in the screenshot below:
Please note, if you don’t see a ‘License’ tab then it’s likely that you still have the free version of the plugin installed. The free version and Pro version are actually two separate versions of the plugin. When you purchased the plugin you should have received a link to download a zip file of the Pro version, along with your license key. This should also have been emailed to you at the address that you entered when purchasing the plugin. The Pro version contains the ‘License’ field which allows you to enter and activate your license.
There are step-by-step instructions on how to install and activate the Pro version here. If you want to transfer your settings from the free version to the Pro version then you can do the following:
1) Check the ‘Preserve Settings when plugin is removed’ option on the plugin’s Settings page
2) Remove the free version
3) Install and activate the Pro version