The control can work in any type of website. MVC is no problem.
You should check at which location you are trying to access the XAP file from your server. You can use Fiddler for that.
Maybe you have to modify the path to the control/XAP-file so it's able to load the Silverlight Multi File Uploader from the correct location.
Modify this line:
<param name="source" value="ClientBin/mpost.SilverlightMultiFileUpload.xap" />