It looks like you are trying to use LINQ to find the index of "HD2011-1010", but I'm not sure LINQ is the easiest way to do that. Instead, you might consider something like this:
Module Module1
Sub Main()
Dim myarray() As String = {"HD2011-1009", "HD2011-1010", "HD2011-1011", "HD2011-1012"}
Dim indexOfInterest As Integer = Array.IndexOf(myarray, "HD2011-1010")
Dim prevIndex As Integer = indexOfInterest - 1
Dim prevElement As String = myarray(prevIndex)
Dim nextIndex As Integer = indexOfInterest + 1
Dim nextElement As String = myarray(nextIndex)
System.Console.WriteLine("NextElement is " & nextElement)
System.Console.WriteLine("PreviousElement is " & prevElement)
End Sub
End Module
Best,
Shawn