In Workspace > Email Marketing > Campaigns, edit the campaign and on the Attach Template step click Use for the template to send.

Select the text box to add the merge fields and double-click the Text content to place it in Edit mode. Select the Mail Merge field from the drop-down list.

NOTE: Mail Merge fields are based on the mailing list(s) attached to the campaign (Attach Recipient List step). For example, if a mailing list were created from the Transactions grid, transaction data will be available. If a mailing list is imported it will only have First Name, Last Name and Email available.