Workbooks is a collection in Excel, not Access. At least, I cannot find any reference to the Workbooks collection for Access on MSDN. Bottom line, your code is using an Excel reference.
I would start by instantiating an instance of Excel and use that instance to open your xlsb file. Apparently, you already have created a reference to Excel in your Access application, otherwise the code you posted would not compile.
So maybe something like ...
Code:
Dim xlApp As New Excel.Application
Dim myBook As Workbook
Dim mySheet As Worksheet
Set myBook = xlApp.Workbooks.Open("C:\Test\ExcelFiles\TestFile.xlsx")