In most cases, when you design a Flash game the main screen is the last thing you add to your project.
This way, sometimes you may forget to include a feature or two because you want your game to be released into the wild.
Since you shouldn’t understimate the main screen (all in all it’s the first thing players see), here it is a brief guide about it. Follow this post and you will be able to create basic Flash game main screen in a matter of minutes.
http://www.emanueleferonato.com/2010/06/17/anatomy-of-a-flash-game-main-screen/