The problem is that you're working with paragraph breaks and inserting tabs after them, rather than working with paragraphs and inserting tabs before them. Try:
Code:
Sub Demo()
Application.ScreenUpdating = False
With ActiveDocument.Range
With .Find
.ClearFormatting
.Replacement.ClearFormatting
.Text = "[!^13]@^13"
.Replacement.Text = "^t^&"
.Forward = True
.Wrap = wdFindContinue
.Format = False
.MatchWildcards = True
.Execute Replace:=wdReplaceAll
End With
End With
Application.ScreenUpdating = True
End Sub