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You will need to contact Melissa Data to purchase a license to access the service. Do you already have a sales contact at Melissa Data?
Accurate as of Sept 2017:
Lauren Sveen (Sullivan)
Account Executive | Sales Operations Lead
lauren.sveen@melissa.com
You purchase a license for either plug-in or both (if you want to use both) and they have limits on volume of data to geocode so they can help you with figuring out what options are available.