If my assumption is correct here's how you do it, you're basically traversing through the listbox and adding the values, easy:
Public sumOfListBox As Integer
Private Sub cmdSumOfListBox_Click()
'Loops through every item from the beginning of the listbox to the last element
For i = 0 To Me.List1.ListCount - 1
sumOfListBox = sumOfListBox + CInt(Me.List1.List(i))
Next
'Displays the final sum in a message box
MsgBox (sumOfListBox)
End Sub