Creating a FLASH game is as easy as creating an HTML5 game. You have just installed the FLEX compiler on your system. If you are on Windows, then you also need to install the JAVA SE development kit. For more detailed instructions about this, please refer to the Getting started section in the Monkey documentation.
After you have installed these software kits, then just restart Monk. When you build your game, you should now see a new FLASH entry in the list of build targets. After a successful build, you will find a FLASH folder inside your build folder, which contains all the files that you will need to copy to your web space if you want to publish your game there.