I think you'll find the following does what you want far more efficiently:
Code:
Sub Highlight_Words()
Application.ScreenUpdating = False
Options.DefaultHighlightColorIndex = wdYellow
With ActiveDocument.Content.Find
.ClearFormatting
.Text = "Procedure Step:[!^13]{1,}"
With .Replacement
.Text = "^&"
.ClearFormatting
.Highlight = True
End With
.Forward = True
.Wrap = wdFindContinue
.Format = True
.MatchWildcards = True
.Execute Replace:=wdReplaceAll
End With
Application.ScreenUpdating = True
End Sub