Using Multi-select list boxes as a RecordSource for a form.

What you need is a comma separated list in a text box which uses each of the 
items selected. Here's an example using a hidden text box named txtSelected:

        With Me!lstElevation
            If .MultiSelect = 0 Then
                Me!txtSelected = .Value
            Else
                For Each varItem In .ItemsSelected
                    strList = strList & .Column(0, varItem) & ","
                Next varItem
                If strList <> "" Then
                    strList = Left$(strList, Len(strList) - 1)
                End If
                Me.txtSelected = strList
            End If

        End With

With this, you change the recordsouce of your form or subform:

Then txtSelected is used in an IN clause in your code:

strSQL = "SELECT * FROM tblItem WHERE (((CategoryID) In (" & Me.txtSelected & "));"

Me.RecordSource = strSQL

or:

Me.SubformControlName.Form.RecordSource = strSQL