ya i have a same issue..
you just right click to add new items-->global application class for global.asax.
then add the class given below..
void Application_Start(object sender, EventArgs e)
{
// Code that runs on application startup
ScriptResourceDefinition myScriptResDef = new ScriptResourceDefinition();
myScriptResDef.Path = "~/scripts/jquery-1.8.1.js";
myScriptResDef.DebugPath = "~/scripts/jquery-1.8.1.js";
myScriptResDef.CdnPath = "http://code.jquery.com/jquery-1.8.1.js";
myScriptResDef.CdnDebugPath = "http://code.jquery.com/jquery-1.8.1.js";
ScriptManager.ScriptResourceMapping.AddDefinition("jquery", null, myScriptResDef);
}
then start debug.
its will surely work..