I found similar problems and found that I had to register to use MobileMe from a PC and not from an iPad. It installs software onto your PC that allows you to configure MobileMe - what do you want to sync between your iPad and PC - Contacts? Bookmarks? etc.
I then installed the iDisk app (free) from the App Store from my iPad and, when it asked for my details, I entered those I'd used when I registered from the PC. It accepted those and now everything works fine.
I can now upload files from my PC to MobileMe. To do this you go to the MobileMe website from your PC (I found Safari on the PC worked best, I normally use FireFox but this gave all sorts of problems), log on and there you are presented with a whole range of options. If you choose the iDisk option you can upload or download files from your PC to MobileMe - MobileMe has a number of default categories - Music, Video, Documents etc - but you can create new folders and sub-folders (a little difficult to find that option and I found it easy to create folders in the 'wrong place', so you need some patience!).
From the iPad iDisk app you can similarly view the files you've uploaded (and, if they're mp3 files play them from within iDisk) and, if the file type is recognised by the iPad, you can open the file from the appropriate app.
Similarly, if you're within an app (say the Apple iWork 'Pages' (MS Word compatible)) then you can directly import or export files from the iDisk (assuming, of course, that you are connected to the Internet). I've not found any problems with it. You also get an email account ('name'@me.com) that will 'push' email, contacts, bookmarks (assuming you chose to have them sync'd).
All-in-all it works fine and it's an easy way to share documents and other files between your PC and iPad without having to go through the tortuous process of connecting the iPad and PC and using iTunes. In fact, it more-or-less replaces the functionality of iTunes except for one very important omission - MobileMe doesn't backup your iPad. So you still need to connect the iPad and PC occasionally to back up.
Tim
Scotland