The README.txt for the Silverlight search widget includes instructions for building the widget project.
Unfortunately it depends on "ESRI ArcGIS Sample for WMS Data Source" but the link to that WMS sample is not valid any more.
Please provide current location where that WMS sample can be found.
Quotes from README.txt showing broken hyperlink...
Required tools for project compilation and testing:
3) ESRI ArcGIS Sample for WMS Data Source (http://resources.esri.com/arcgisserver/apis/silverlight/index.cfm?fa=codeGalleryDetails&scriptID=16249)
To compile/run this tool you need to.
3) Download ESRI ArcGIS Sample for WMS Data Source source code from the esri code gallery and unzip it to a directory;
4) Copy the GeoportalWidget folders in this directory into the parent directory of the above unzipped resource;
5) Create a solution using Visual Studio 2010;
6) Create two class library projects in the solution. These two projects correspond to two folders created in steps 3 and 4. Add the corresponding c sharp files into these two projects.
7) Add references to WMS project in the GeoportalWidget project