In order to link the Membership to a Gift, you must already have saved a Gift mapping as well as a Membership mapping.  With both of those mappings in place, you can create the mapping for the Gift Link. You can only link to a new Gift being added via your template.

While both fields on this mapping are required, you have the option for the Applied Amount to be either a Source Field or a Default Field.  For the Import/Linked With, you can only select the name of a Gift Mapping from the dropdown menu.