You can't. At least not yet, and I don't know that you'll be able to on the S1 at all. If you search the blogs you'll find a post about the Blackberry developers conference that outlines the major advances we have to look forward to in the coming months. Native PDF support, a webkit browser, OpenGL and Flash support.
That's the good news, now the not so good news... I can't specifically remember, but I don't think that any of those "goodies" will be available to owners of the first generation Storm. I think that the S1's processor and available memory aren't sufficient. Like I said earlier, search the blogs (not the forums) for posts/articles pertaining to the Blackberry developers conference. There is one post specifically dealing with Flash support...
Happy hunting, and think about upgrading to the S2, you won't be sorry!!!