All your custom layouts can be accessed via VBA through the CustomLayoutscollection of the SlideMaster property of a Presentation object. When you create a custom layout, give it a meaningful name. Then you can fetch it from the CustomLayouts collection. It appears that Microsoft didn't implement lookup by name, so you will have to iterate through the collection to find the CustomLayout object with the right name.
Once you have a reference to the desired CustomLayout object, you use the AddSlide method of the Slides collection, which takes a CustomLayout object as the second arguments (as opposed to Slides.Add, which you used in your question, and which takes a PpSlideLayout enumeration value).
Below is a helper method for fetching a custom layout by name, and example of using it as you wanted:
Public Function GetLayout( _
LayoutName As String, _
Optional ParentPresentation As Presentation = Nothing) As CustomLayout
If ParentPresentation Is Nothing Then
Set ParentPresentation = ActivePresentation
End If
Dim oLayout As CustomLayout
For Each oLayout In ParentPresentation.SlideMaster.CustomLayouts
If oLayout.Name = LayoutName Then
Set GetLayout = oLayout
Exit For
End If
Next
End Function
Sub AddCustomSlide()
Dim oSlides As Slides, oSlide As Slide
Set oSlides = ActivePresentation.Slides
Set oSlide = oSlides.AddSlide(oSlides.Count + 1, GetLayout("Smiley"))
End Sub