Hi Katy,
I'm having a similar issue. I am trying to add at least two WMS layers to my web application that I'm building with ArcGIS Viewer for Silverlight. I've dabbled with trying to program an add-in but I'm not certain how to begin with this. The WMS's are not affiliated with our office, so I don't dare post them on ArcGIS Online. Any help would be appreciated, thank you.
-Courtney