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.
Nevron Support Team