If you want to set certain Outlook fields when you do a mail merge, you need to use an ItemSend rule.

To use, open the VBA Editor using Alt+F11 and expand Project1 until you see ThisOutlookSession. Double click on ThisOutlookSession and paste the macro in the right pane.

Updated July 26 2017: Changed the macro to use a codeword in the subject and remove the codeword before sending.

This macro is not limited to mail merges; it checks the subject of every message you send to see if it meets the condition of the If statement.

To avoid running the macro on all messages you send, I'm using a codeword in the subject. If the word is found, Outlook removes it from the subject and runs the macro on the messages, adding attachments and setting other fields as needed.

If you want to change the codeword, choose something unique that you would not expect to use in an email. Remember: the codeword is case-sensitive. What you use in the macro is also what you'll use in the subject field.

If you are setting a reminder, you'll want change the reminder date and time. You can set the reminder to a specific date and time using #10/30/2012 8:00:00 AM# format or use Now + 1 for this time tomorrow (or any number of days in the future). If you use the deferred delivery option, you can use SendAt + 1 to set a reminder for 24 hours after the deferred send time.

Recipients may have rules to remove the reminder, flag, and Importance fields.

Private Sub Application_ItemSend(ByVal Item As Object, Cancel As Boolean)

If InStr(LCase(Item.Subject), "[merge]") Then
'Defer delivery time
   SendAt = DateSerial(Year(Now), Month(Now), Day(Now) + 1) + #7:00:00 AM#
With Item
      .Subject = Replace(.Subject, "[merge]", "")
      .Importance = olImportanceHigh
      .VotingOptions = "I accept;I decline;I don't care"
      .ReminderSet = True
      .ReminderTime = SendAt + 1 '#10/30/2012 8:00:00 AM#
      .FlagRequest = "Please reply by " & .ReminderTime
      .DeferredDeliveryTime = SendAt
' to add an attachment, enter the path & name here
     .Attachments.Add "C:\Users\username\Documents\TEST.xlsx"
End With
  End If
End Sub

If you want to use a different attachment for each person, you'll need to use attachments that are the same name as a message field - the To field will probably be the easiest, although prone to problems. ("To" is the display name, not the email address.)

Item.Attachments.Add "D:\For merge\" & Item.To & ".docx"

Or use one of the utilities listed at Using Mail Merge in Outlook

Video Tutorial