To get started, visit http://www.acquia.com/downloads and grab the latest, greatest version of the software. Install it using the provided wizard.
Then open the Dev Desktop Control Panel and start the Acquia Drupal Stack.
Full disclosure, the Demo Framework is a big distribution and requires a memory boost. Open the "Settings..." tab and click on "Config"
Demo Framework also runs best on PHP 5.4+ so switch your default version to PHP 5.4.8