After installation of your Live Essentials programs is complete, you can find them by left-clicking on the Start button, then left-clicking "All Programs" at the bottom. You'll see a new folder called "Windows Live". Click on the folder and you'll see all your installed programs. Click on any program to fire it up.
Note that you'll need a Windows Live ID to use the programs. If you already have a Hotmail, Messenger or Xbox Live account, you already have an ID. If not, you can sign up for an ID on the Windows Live Essentials site. Click "Sign in" at the top right, then choose "Sign up" on the next page.