.Net Visual Basic application cant get NPOI to resolve IWorkBook is not defined error [closed]

I have a Visual Basic .NET application with the following code:

Dim ds As New DataSetDim workbook As IWorkbookDim worksheet As ISheetDim sheetCount As IntegerDim file = New FileStream(xlsFileName, FileMode.Open)workbook = New HSSFWorkbook(file)sheetCount = workbook.NumberOfSheetsDim table As New DataTableworksheet = workbook.GetSheetAt(0)

The IWorkbook, ISheet, and HSSFWorkBook all generate an "is not defined" error message.

I have included NPOI (2.6.2) in my packages as well as Microsoft.Office.Interopt.Excel (15.0.4795.1001).

What other packages am I missing?

Thank you for any insight.