Well, first of all, how long do you have? Giving a 15-minute presentation is very different from giving a 1-hour one. Are you supposed to teach actually useful techniques, or just introduce the topic for an unfamiliar audience?
I would start by gauging your audience. You don't want to spend your presentation addressing concerns they don't actually have, or talking about things they already know. Don't you think people who go to a conference called "Script and Digital Games" are already open to the idea that games can have good scripts? And since they're professional scriptwriters, they probably have the basic knowledge about how scripts are written, and so on.
Also, think about the other presenters and what they will have talked about. Are you the only one representing the gaming side? Should you stick to adventures or cover games in general?
Maybe you can start off by showing a clip from a game with a cinematic look and high production values (and a decent script). For example: Fahrenheit, The Pandora Directive, Blade Runner or Dreamfall. You can show how playing it two different ways produce different scenes, both of which need to work dramatically. (Fahrenheit has several excellent examples of this in the early chapters.) It's a pretty basic point, but to people used to dealing with a linear story it's probably one of the biggest differences. (Blade Runner would also let you make a number of points about how different interpretations and versions of the film have been incorporated in the different paths and endings of the game.)
Once you show that games can look like movies, I don't think it'll be hard to make them realize that the two have things in common. From there, I think you should quickly move on to how you actually write an adventure game script, and especially how you solve some of the unique challenges (dialogue for all kinds of generic actions, room-specific actions that can be performed at any point in the story, repeated actions, dialogue trees, etc.). If you use any tools like mindmaps, wikis, or anything like that, show it! And you can also talk about how having the player perform specific actions compares to watching a character do it in a movie.
Personally, I'd also be interested in the topic of what it means for the audience involvement that they're playing a game, and working very deliberately towards "winning" (which can make some dramatic devices problematic). You don't have to talk about it, of course, but the ending of Diamonds in the Rough would probably provide some interesting material for that theme. (I wouldn't worry too much about spoiling the game for them. If you convince them that they can learn from you they'll probably buy it anyway.)