Engadget published a guide to run Flash games on your Xbox 360 but they also point out that this doesn't work anymore on updated systems because Microsoft released a Dashboard upgrade yesterday which prevents you from doing this.
The first thing you are going to need to do is find a copy of the kiosk disc. We can't tell you where to get it , but check your favorite BitTorrent tracker and you'll find the 4.1GB release (it is labeled PAL, but works fine on US boxes). It has been out for a month so it shouldn't be encumbered by some obnoxious private tracker. To boot Flash files you don't even need the entire disk. So your best bet is to look for just the Hexic files from the disc, which are only 60MB.