You can get either the old "MailMergeHelper" or just the Header file related
commands back by selecting them in Tools|Customize|Commands|category All
Commands and dragging them to a suitable toolbar.

However, you may find that Header sources do not work quite the same as they
do in previous versions of Word - e.g., you may not be able to attach /just/
a header source and then use the built-in drop-downs to insert merge fields,
and you may not be able to use a Header when you use Access/Excel data
unless you modify the connection method to the old default, DDE (check Word
Tools|Options|General|Confirm conversions at open, then reconnect tot he
data source and select the DDE option when offered.)

Peter Jamieson