Installing Drupal with existing Pantheon site? First, I am completely new to, well, all of this... So I'm sorry if I sound like an idiot. I have been fighting with this thing all day, and I think I'm more confused then when I started. I work for a non-profit, and I'm supposed to be helping our web designer with the new website. He uses Drupal and Pantheon. His instructions were something to the effect of "Download the repositories from Pantheon, install Drupal, and point Drupal to the repositories" Yeah... I downloaded the stuff from pantheon using Git, so I think I at least did that part right, but I have no idea how to install Drupal. I have read half a dozen different manuals, all of which tell me to do different things, most of which I don't understand. And our web designer can't help me because he doesn't know how to do any of this on a Windows box. (He's a Linux guy). Help??

I'm using Windows 7 Ultimate.
To install Drupal on an existing Pantheon site you don't need to install or copy anything to your local computer; you can just work on the Pantheon development environment in the cloud using the dashboard workflow. Pantheon hosts three distinct environments for each site: development, testing, and the live production site. You can install Drupal plus modules and themes on your development site. Once you're satisfied with the results, you can push your site code to the test environment for user testing and then to the live environment to serve the site from your domain, once you've launched with a paid site. The workflow tools let you push code from dev->test->live, and also clone your database and files between environments. Typically you'll install Drupal and configure the modules on the development environment before launching the site, then clone that database up to live to retain all the content changes. However, post-launch you would only want to copy the live database and files down to development and testing environments, as the live production environment is the canonical source of site content. You'll want to review Pantheon docs for how to use the workflow, both pre-launch and post-launch:

https://pantheon.io/docs/articles/getting-started/