You would issue it as an instruction in a script.
Let's say you had an object in your scene, say a book, and this was shown as a graphic in a node called 'book'. Then let's say you also had a video of the book opening, called 'BookOpening.ogg'
Then attached to the 'book' node, you might have a script 'book.script'. Inside that script would be what would happen if your player clicked on the book:
on "LeftClick"
{
Game.PlayTheora("BookOpening.ogg");
}
When the player clicked on the book, your video would run, and your player wouldn't be able to do anything else until it had completed.
It's all well described in the docs.
Cheers
