When you have installed and enabled the plugin and you have setup the custom field, you can start using it.
Go to the article you like to add the Slideshare Presentation.
You see a new tab. In this example it's called "Video" but it's the name you choose for the the Custom Fields Group.
You also see the description if you entered one in the Custom Fields Group.
Slideshare: Here you enter the WordPress Shortcode of the Slideshare Presentaion.
You can find the WordPress Shortcode of the Slideshare Presentation after you click the share button in Slideshare