You can't open files into Notepad++ directly from the files app. You will need to open Notepad++ first and then open your file via the menu.
Anything in your Google Drive won't be accessible, so anything you want to edit will need to be moved to your Downloads folder. I advise you create another Downloads folder inside your Downloads folder and change your chrome settings to save downloads there. This give you a clearer view in your main Downloads folder to add extra folders and store files for editing.
When opening files via the app you will need to select 'My Android Downloads'. This is your main downloads folder. All other available folders are folders inside crossover. They exist to simulate a windows environment for the windows applications (I think so anyway, I'm not a crossover developer). Simply put, the other folders aren't accessible from anywhere else outside of the Notepad++ application and therefore saving files in there will be of no use to you.
At this current time the only way to edit files in your Google Drive is to copy them into your Downloads folder, edit them and then copy them back into Google Drive. I don't believe this is due to crossover but rather due to how chrome os handles android apps and their file systems.
