When using the Membership data destination, Importacular will determine the Membership Type based upon the hierarchy of Memberships within your instance of Importacular. Since this is determined by your setup, we do recommend adding this hierarchy prior to importing any Memberships using Importacular.
To set this up, you will use the Application Settings (found under the File menu).
Once you open that, you can click on the tab for the Membership options and configure your settings there.
Make sure that you rank your Memberships from lowest to highest as that is how the system will determine the Membership Type.