Unzip into your /wp-content/plugins/ directory. If you're uploading it make sure to upload
the top-level folder. Don't just upload all the php files and put them in /wp-content/plugins/.
Activate the plugin through the 'Plugins' menu in WordPress
Visit the options page (Options - I am reading) for Wordpress 2.3.x, (Settings - I am reading) for Wordpress 2.5.x-2.7
Choose the source (AWS / OpenLibrary) and setup an ISBN.
Activate the sidebar widget or insert iar_print_html() in your template.