The Foliobook startup assistant has a 'buy' button, but if you don't purchase the plugin from there you can find a new plugins menu

Select the 'F' toolbar icon then Plugins... this will allow you to see and purchase the Video Plugin, firstly select the 'Plug Ins' menu item

Plugin menu

Any plugins available, or already purchased will be listed here. In this version of Foliobook only the Video plugin is available

plugin menu 2

Selecting an available plugin shows a description of the plugin and a buy button. The price will be populated from your relevant app store in your local currency

video store