Use OnKey. Do one of the following:
1) Place this in the ThisWorkbook module (this will take over the enter key when you open the workbook and put it back to normal when you close):
Private Sub Workbook_BeforeClose(Cancel As Boolean)
    Application.OnKey "~"
End Sub
Private Sub Workbook_Open()
    Application.OnKey "~", "AutoIncrement"
End Sub
--OR--
2) Add this to a module and call CatchEnter it to toggle the behavior:
Public blnCatchEnter As Boolean
Public Sub CatchEnter()
    If blnCatchEnter Then
        Application.OnKey "~"
        blnCatchEnter = False
    Else
        Application.OnKey "~", "AutoIncrement"
        blnCatchEnter = True
    End If
End Sub
Next, add this macro to a module:
Public Sub AutoIncrement()
    If TypeName(Selection) = "Range" Then
        Dim lngRow As Long
        lngRow = Selection.Row + 1
        Range("A" & lngRow).Formula = "=ROW()-7"
        Range("B" & lngRow).Select
    End If
End Sub
Although, I agree with @BruceWayne that you may not want to take over the Enter key this way. Assigning a shortcut to AutoIncrement will be safer.