MobileSheets does not support folders, as I have no desire to recreate something like a file system with an infinite level of depth, as it's slow to navigate. I know this is a divisive topic for some users that love having many folders, but the absence of folders or anything like that is intentional. If you want to use MobileSheets, you'll have to change the way you manage the organization, filtering, sorting, etc to utilize metadata for your grouping instead. You can have the option to manage all the files on the device however you would like though.
Mike
Mike