Upload the folder containing iframe-widget.php to the /wp-content/plugins/ directory
Activate the plugin through the 'Plugins' menu in WordPress
To add an IFrame on a sidebar, browse to Design > Widgets and add the 'IFrame Widget" to desired sidebar. Configure the IFrame Title, Dimensions and URL and save your changes.
To add IFrames to any post or page just add the markup [dciframe]url,width,height[/dciframe], for instance [dciframe]http://www.google.com,400,250[/dciframe].