In Workspace > Email Marketing > Campaigns, edit the campaign and on the Attach Template step click Use (Use Template) for the template to send with the email campaign.
At this point, double-click Text content to place it in Edit mode and select the Mail Merge field(s) 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 in the
campaign than the individual mailing lists themselves.
Summary:
A simple email list or imported list will only include the First Name, Last Name and Email mail merge fields.
If a mailing list is created from a grid using Actions > Create Email List, additional mail merge fields will be available. The data (except for name and email) on this type of mailing list is static. If updated data is needed, a new mailing list should be created.
Attaching multiple mailing lists to an email campaign may affect which mail merge fields are available as well.
In Workspace > Email Marketing > Contacts, select the mailing list and run Actions > Export to Excel File to see which mail merge fields are available, and the data populated for the mail merge fields.