You can’t use plugins on a WordPress.com blog. Please not that it says ” If you are a WordPress.org user and would like to use this feature on your self-hosted blog, it is available as a plugin.” WordPress.com is not for self-hosted blogs. You need to understand the difference between WordPress.com and WordPress.org:
If you are posting source code to show someone how you did something – you will need to wrap it in the various short code parameters given at the link you posted. Sort of like programming or using a real old test editor.
You will need to sort of wade through the options and do some experimenting