You don't need SSAS installed on your computer, but you do need to have the driver installed, which sounds like you've already done. Curious, what driver did you download?
I have seen the message 'file being used by another process' when I have a connection open to the file in either another window of Tableau Desktop or Excel. I'd double check that there's nothing else using the file, it may be easier to create a copy on another location and connect to it to be safe.
I downloaded the cube file and driver from the connecting to cubes training video page on tableau website. I actually installed all of the above on a clean virtual machine. The cube file had never been opened by another program. I unzipped it and copied it to the virtual machine.
Did you ever get this to work? I have exactly the same question and I also did not open this file before either.