The Maskon CD-ROM? It doesn't exist yet, but we asked Mike Werb, the movie's scripter, to test-drive StoryVision (310-392-5090, $199), new PC software to help write an original interactive program. Here's what he uncovered. Everyone who's seen The Mask has said to me, ''Wouldn't it be great if Stanley had done this?'' Or, ''I would have loved to see Milo do that.'' So, with StoryVision I had a blast adapting my script into a multimedia game. And though I've used computers only as glorified typewriters, I found StoryVision surprisingly easy to follow. I felt like I was actually playing a game while I was creating one. Usually when I work on a script, I write each scene on an index card. I end up with 60 to 120 cards and then lay them out like a movie. StoryVision lets you draw a diagram of all the scenes, describe each one in its own balloon, and then designate the way they connect, almost like a family tree. This branching structure, which helps you present different options that a player can choose during the story, makes it clear where those options should come up. I started with Stanley finding the mask. Next, a player could have Stanley put the mask on or give it to his dog, Milo, as a chew toy. Milo might bury the mask in the yard, unearthing Jimmy Hoffa's corpse in the process. There are a couple of things that took a little getting used to. Because StoryVision allows you to write zillions of subplots, you might get confused, since many of the branches will be hidden-you have to get used to the fact that not everything is on the screen at once. But would I write another interactive game? Somebody stop me!