There are two ways: one is to explicitly cast the object you retrieve from the ArrayList back to a Details structure:
Dim detail As Details = TryCast(strucStatementInfo.strMaster(i), Details)
If detail IsNot Nothing Then
With detail
...
End With
End If
But the better way is not to use an ArrayList at all: use a generic List instead:
Dim myList As New List(Of Details)()
Because it will only accept objects which are Details structures, and you don't need to cast the object when you take it out becuse the system already knows what it is.
[edit]Lack-of-caffeine induced spelling errors - OriginalGriff[/edit]