It seems you have early version fo DNN 5 where ToJson is not implemented yet.
To Fix your site you have to
- Go the DNN files root and open App_Code\XsltDb\XsltDbController.cs
- Find definiton of XsltDbExtensions class
- Add the following extension method declaration
public static string ToJson(this object source)
After that your site should be OK.
ASAP I'll issue new release that will not contain ToJson and test it on DNN 5.0 to ensure it is supported.