The package Microsoft.Office.Interop.Excel may not be available in the xaml by default. So, try importing the package from the manage packages then use the below code.
Here is the sample code …
Dim excel As Microsoft.Office.Interop.Excel.Application
Dim wb As Microsoft.Office.Interop.Excel.Workbook
Dim ws As Microsoft.Office.Interop.Excel.Worksheet
excel = New Microsoft.Office.Interop.Excel.Application 'create the instance of excel work book'
wb = excel.Workbooks.Open(" Path to the file") 'Open the excel the file
excel.Visible = True
ws = CType(wb.Sheets("Sheet Name"), Microsoft.Office.Interop.Excel.Worksheet) 'select a sheet and activiates'
ws.Activate()