Using Markdown on WordPress.com
The instructions from this guide are referring to the WP Admin dashboard. You can get to this dashboard by adding /wp-admin to the end of your site’s url (e.g.: example.wordpress.com/wp-admin)
Markdown can be enabled for posts and pages in your dashboard, under Settings → Writing, and for comments, under Settings → Discussion. Once enabled, it will be available to all users on your blog. To enable Markdown for posts and pages, go to Settings → Writing in your dashboard, and check the box labeled Your posts will be run through the Markdown text processor. Click on Save Changes at the bottom of the page to apply. You will now be able compose posts and pages using Markdown.
To enable Markdown for comments, go to Settings → Discussion in your dashboard, and check the box labeled Comments will be run through the Markdown text processor. Click on Save Changes at the bottom of the page to apply. Visitors to your blog will now be able to compose comments using Markdown.
See this page for more information on using Markdown while writing posts.