The companion app does not support, and has never supported, importing through cloud services. Just download the files first onto your device using a browser, then you can import them into the companion app. At some point I may explore supporting cloud services, but it's fairly complicated and I would have to go through entirely new approval processes with each cloud service as it's a separate app, and I'm not keen to spend time and effort on that.
If you absolutely need access to them on your Windows device, you'd have to purchase the full Windows version of MobileSheets instead, then import things into the library, then synchronize changes to the iPad.
Also, I should add that you can just import directly on your iPad if you are using cloud services. Just tap Import->Dropbox/Google Drive/OneDrive on the iPad and select the files off the cloud. You don't really need the companion app if that is your plan.
Mike
If you absolutely need access to them on your Windows device, you'd have to purchase the full Windows version of MobileSheets instead, then import things into the library, then synchronize changes to the iPad.
Also, I should add that you can just import directly on your iPad if you are using cloud services. Just tap Import->Dropbox/Google Drive/OneDrive on the iPad and select the files off the cloud. You don't really need the companion app if that is your plan.
Mike