Accessing 3rd party app files in iCloud from Windows
I don't think one is able to access 3rd party app data via www.iCloud.com. You may have a couple options despite that, the first being to get a Mac. Honestly, that's probably not very practical for you. Otherwise, IAWriter, may have a way to log into your account there and access files - a feature not uncommon with apps like that. If neither of those options work, then you will probably have to wait until they put out an app for Windows (which may not be very high on their list if it is on it at all.