Make sure you have copied your license/trial key from Avrodh.com. Please check this topic for more details.
Open Avrodh - Web Extension settings.
Click on "Purchase" menu item.
Click on "Activate" button.
You will now see "Activate License" dialog. Please enter (or paste) your license/trial key. Click on checkbox and then click on "Activate" button.
Extension may ask permission to validate license/trial key. In a few seconds your license will be activated. If there are any errors, message with more details will appear on screen.