Hello Petete,
The Store Manager has a built in HTML editor allowing to add a embed youtube video:
http://screencast.com/t/dTwfgDmNoLAh
In case you prefer to work without the HTML editor, you may use the following:
<p><iframe src="http://www.youtube.com/embed/vHo9IhbYbog" frameborder="0" width="620" height="349"></iframe></p>
Here is the example from our store:
http://store.emagicone.com/store-manager-for-prestashop-pro-edition.html#product_tabs_custom1