Hi there Max
Let us just and keep things separated. This guy is trying to get the USB connection working on Ubuntu so he can manually to transfer files to his brick. He is not using the plugin. In order to achieve this he must get the RNDIS driver working on his installation. I posted a link that should help him get started.
My closing comment about using WiFi has nothing to do with Ubuntu. If the brick is connected to a WiFi network you can communicate with it from either Windows (using Putty), Mac or Linux/Unix over SSH and easily transfer files. So your comment about WiFi not working makes no sense… I guess that you where talking about the Xamarin plugin?
Your claim about the plugin not work on Linux/Ubuntu is simply not true. As you correctly says Xamarin Studio does not exists for Linux/Unix. On these systems you use MonoDevelop. Since Xamarin Studio is a rebranded version of MonoDevelop that uses platform-specific code to enhance the look and feel on Windows and Mac OS the plugin for MonoBrick works without problem with MonoDevelop. Just to show what it looks like on Linux I have provided a few screenshots.