In Workspace > Email Marketing > Campaigns, edit the campaign and on the Attach Template step click Use for the template to send.
At this point, 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 Gifts grid, gift data will be available. If a mailing list is
imported it will have First Name, Last Name and Email available. If multiple mailing lists are attached to
the campaign, fewer mail merge fields may be available than the individual mailing lists.