The find string you are currently using already "finds" paragraph marks. If what you tried to do worked, it would only find the starting keyword if it occurs at the end of a paragraph. Is that what you want?
Code:
Sub ScratchMacro()
'A basic Word macro coded by Greg Maxey
With Selection
.HomeKey Unit:=wdStory
With Selection.Find
.ClearFormatting
.Replacement.ClearFormatting
.Text = "Start_Word" & Chr(13) & "(*)End_Word"
.MatchWildcards = True
.Replacement.Text = ""
.Execute Replace:=wdReplaceAll
End With
End With
lbl_Exit:
Exit Sub
End Sub