From version 1.12.5, the “Import Data” menu has been removed. The functionality is still available, though.
Import invoices
For bulk creation of Invoices, just go the the admin page to add a new Invoice. Instead of creating a single Invoice for one specific Member, you can select a Member Category and create a batch of new Invoices by following the steps in the wizard at the bottom of the page.
Import Members
Similarly, import Members from a CSV file by using the admin page to add a new Member. At the bottom of the page, a CSV file can be entered to upload an existing member list to your site.