Upload greader-blogroll.php to the /wp-content/plugins/ directory.
Activate the plugin through the 'Plugins' menu in WordPress.
Got to Design -> Widgets in your dashboard, ADD the "Google Reader Blogroll" to your sidebar.
You need to specify the link to your public Google Reader page, can be found in the Google Reader settings: https://www.google.com/reader/settings in the "Folders and Tags"-tab, the "view public page"-link. Just copy/paste the whole link.
The widget allows you to choose one of Googles cholor schemes, if you select "None" it will generate a generic list that should look quite fine with most themes.