Extensions are themselves WordPress plugins and if you purchase one or have one included with your license you can download and activate it to WordPress.
Every activated extension will appear on the AFP License page where you will be able to add your License Key for it. If your AFPv2 license includes the extension you would use that (Professional License for Pro Extensions or Managed License for Pro & Premium Extensions).
After a plugin is activated with its license it has to be enabled which is done on the AFP Extensions section. This allows you to turn on / off the features of an extension even if it is activated.