To set up a Lost Customer message, navigate to Communications, and Campaign Manager.
Click on your default campaign, then click on Add Media to create a new message, or copy an existing campaign message (Read more about how to do this)
Once you've crafted the perfect message to motivate your customer to return, you can add to the campaign. At the bottom of the campaign message list is the "Add media" area.
Select the new message you created. Select the "Lost Customer" trigger. The X value will be the # of days since the last transaction. The Y value is the expiration date of the offer.
Once you've linked to the campaign, be sure to click on the offer column to add an offer.
You can set up multiple "Lost" emails with escalating offers as the last visit date gets further and further out. You'll definitely see results in bringing back those customers who may have forgotten about you!