Hi Mike,
Generally there are 2 types of mail merge fields - predefined mail merge fields and source mail merge fields. If by custom fields you refer to the source mail merge fields (the ones that come from your data source), then you can insert them from the
Insert Merge Field drop down of the
Mailings ribbon tab as shown on the following screenshot:
You can also insert them by clicking the
Insert Merge Field button directly (not the drop down) and selecting
Mail Merge Source Field for field type in the dialog that opens.
Best Regards,
Nevron Support Team