There's a Submit button right next to the News button, which is next to the Community button. I'm surprised you haven't noticed it. When you submit a game, make sure it's your own!
You get the file by saving your Flash .fla file as a .swf. However, if you are a Flash developer, you should probably already know this. If you are trying to submit a file that you have not made, don't. Only submit your own games ;-) thanks!