Using custom entity to add existing leads to a marketing list

Hi,

I'm trying to use the blog post here as a guide for quickly adding leads to a new marketing list without creating duplicates.  http://blog.clickdimensions.com/2014/10/create-a-marketing-list-from-an-imported-list-of-event-attendeeswithout-creating-duplicates.html  Can someone walk me through the process of correctly setting up the custom entity that I need to have in place in order for these instructions to work?  Thanks very much!

Laura