While Paul is correct, if you are copying the content of the named document into the active document then the following will do that. You don't have to use the clipboard (though you could).
Code:
Dim oDoc As Document
Dim oSource As Document
Dim oRng As Range
Set oDoc = ActiveDocument
Set oSource = Documents.Open(Filename:= _
"X:\sconner\CCD Damage Mechanisms\Mechanisms\Underdeposit Corrosion.docx", _
ConfirmConversions:=False, ReadOnly:=False, AddToRecentFiles:=False, _
PasswordDocument:="", PasswordTemplate:="", Revert:=False, _
WritePasswordDocument:="", WritePasswordTemplate:="", Format:= _
wdOpenFormatAuto, XMLTransform:="")
Set oRng = oDoc.Range
With oRng.Find
Do While .Execute(FindText:="(NEXT MECH)")
oRng.FormattedText = oSource.Range.FormattedText
oRng.Collapse 0
Loop
End With
oSource.Close 0