Loop through the ItemsSelected collection of the unbound multi-select list
box, to build up the WhereCondition of the OpenReport.

Details and example code in this article:
   Use a multi-select list box to filter a report
at:
   http://allenbrowne.com/ser-50.html
If no items are selected, the WhereCondition will be blank, and so all items
will be shown in the report.

The code below clears all selections in the list box:

Function ClearList(lst As ListBox) As Boolean
On Error GoTo Err_ClearList
   'Purpose:   Unselect all items in the listbox.
   'Return:    True if successful
   Dim varItem As Variant

   If lst.MultiSelect = 0 Then
       lst = Null
   Else
       For Each varItem In lst.ItemsSelected
           lst.Selected(varItem) = False
       Next

   End If

   ClearList = True

Exit_ClearList:
   Exit Function

Err_ClearList:
   MsgBox "Error " & Err.Number & " - " & Err.Description
   Resume Exit_ClearList
End Function