Download the zip from here or from Github and drop it into your site's wp-content/plugins directory.
Navigate to your site's Admin->Plugins section (wp-admin/plugins.php) and activate the plugin.
Go edit a post/page/custom-post-type, or add a new one, and look for the ACE tab next to the Visual and HTML Mode tabs at the top right of the content editor box. If you don't see the Visual tab, that's great! It means you or someone else disabled Visual Mode. That'll make toggling between modes a much more pleasant experience (see more in release notes).
Requires: 3.4 or higher Compatible up to: 3.5.2 Last Updated: 2012-12-14 Downloads: 1,003