Activate unsaved workbook from another workbook

Hello,

I am trying to activate an unsaved workbook from a seperate workbook. The unsaved workbook is a report that is exported from another program into excel, I want to reference this workbook from a macro in a seperate workbook. I have tried

Windows("Book1").Activate
Workbooks("Book1").Activate

I have tried Dim wb as Workbook and then assigning Book1 to be wb, same with the worksheet within book1. Every time I do this i get the subscript out of range error, which I know means I am attempting to reference something that the macro isn't finding. I am thinking this might be because the two workbooks are seperate instances of excel? If anyone has any ideas let me know! Thanks!!

Kendra