Multiselect listbox in Access 2010 - ms-access

Question

So I have this search form that filters projects based on the designer type. The designer can be internal, external, or combination. It was originally done using a combobox. I was asked to make it so that the user can filter by multiple options, so I changed it to a listbox with the idea of using the multiselect option.
The listbox works fine, until I set the multiselct option to anything besides none. Then, all the projects are returned instead of the ones that are selected.
I'm not very experienced in Access or VBA so any assistance would be greatly appreciated. The following is the code for this section.
Private Sub toDesignerExcel_Click()
Dim db As DAO.Database
Dim rs1, rs2 As DAO.Recordset
Dim sSQL1, sSQL2, SourceExcel, FileName, Path As String
Set db = CurrentDb
sSQL1 = "Select ExcelPath from tblBackendFiles where Setting = 'ExcelDesignerParameters'"
sSQL2 = "Select Setting from tblBackendFiles where Code = 'SourceExcel'"
Set rs1 = db.OpenRecordset(sSQL1)
FileName = Nz(rs1!ExcelPath, "")
Set rs2 = db.OpenRecordset(sSQL2)
SourceExcel = Nz(rs2!Setting, "")
Path = SourceExcel + "\" + FileName
rs1.Close
rs2.Close
db.Close
Set rs1 = Nothing
Set rs2 = Nothing
Set db = Nothing
Shell "C:\WINDOWS\explorer.exe """ & Path, vbNormalFocus
End Sub
EDIT:
And also,
If Forms(formName).txtDesigner <> "" And Not IsNull(Forms(formName).txtDesigner) Then
If selEngConditions <> "" Then
selEngConditions = selEngConditions & " AND "
End If
selEngConditions = selEngConditions & "[Activity].[GWPDesigner] = '" & Forms(formName).txtDesigner & "'"
End If

Answer 1

I would do something like this :
Private Sub toDesignerExcel_Click()
Dim db As DAO.Database
Dim rs1, rs2 As DAO.Recordset
Dim sSQL1, sSQL2, SourceExcel, FileName, Path As String
Set db = CurrentDb
sSQL1 = "Select ExcelPath from tblBackendFiles where Setting = 'ExcelDesignerParameters' And [Tblbackendfiles] = '"
sSQL2 = "Select Setting from tblBackendFiles where Code = 'SourceExcel' And [Tblbackendfiles] = '"
For Each it In Me.ListBoxName.ItemsSelected
Set rs1 = db.OpenRecordset(sSQL1 & it & "'")
FileName = Nz(rs1!ExcelPath, "")
Set rs2 = db.OpenRecordset(sSQL2 & it & "'")
SourceExcel = Nz(rs2!Setting, "")
Path = SourceExcel + "\" + FileName
Shell "C:\WINDOWS\explorer.exe """ & Path, vbNormalFocus
rs1.Close
rs2.Close
Set rs1 = Nothing
Set rs2 = Nothing
Next it
db.Close
Set db = Nothing
End Sub
Where you basically iterate over all the selected items in the listbox. This adds each selected item from the listbox in the where clause of the query.
You can also try concatenating all the values and changing the query to something like :
And [Tblbackendfiles] In (" & comma_separated_list & ")