If your ListBox names List1, just Add the following code into the correct places (indicated in the commented lines).
Code:
'In the General Declarations
Private Declare Function SendMessageLong Lib _
"user32" Alias "SendMessageA" _
(ByVal hwnd As Long, _
ByVal wMsg As Long, _
ByVal wParam As Long, _
ByVal lParam As Long) As Long
Public Const LB_SETSEL = &H185&
'Into a CommandButton Click event or something you want
Call SendMessageLong(List1.hwnd, LB_SETSEL, True, -1)
I hope this can helps you. Good Look!