First upload to wp-content/plugins and click activate from the WordPress plugins menu.

To restrict an entire post or page, simply select the user level you’d like to restrict the post or page to from the drop down menu added just below the post/page editor.

To restrict just a section of content within a post or page, you may use shortcodes like this:

[restrict userlevel="editor"] . . . your restricted content goes here . . . [/restrict]
Accepted userlevel values are:

admin
editor
author
subscriber