The Ubuntu Windows application does not connect automatically.
First of all you need to add your PC to your account which I assume you have already done as it is looking at the C: Drive
When you right click on the status icon by the clock you should see Status : Idle.
Click on this and it will change to syncing.
Only after this point will it create a UbuntuOne folder.
This should then allow you to play around with your files 'cross-platform'.
There are a few limitations to the public Beta however based on the recent changes to the home page and the removal of the Beta link from one.Ubuntu.com you can assume it will be released or at least made RTM very soon.