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.
Custom mailing lists can have custom mail merge fields imported.