Following is the web site environment I created:
I got it, its given in ScriptManager class itself.
System.Web.UI.ScriptManager.GetCurrent(this.Page);
Have you checked out this:
Working with ASP.NET Master Pages Programmatically
http://msdn.microsoft.com/en-us/library/c8y19k6h.aspx
Not sure what exactly you want here... you want to access the script manager of your master page or just the masterpage?
You can access the master page by
Page.Master in code behind
Or
using System.Web.UI;
ScriptManager ScriptManager1 = (ScriptManager)Page.Master.FindControl("ScriptManager1")
// to access the script manager of master page
or
Page.Master.Controls.Add(Your UserControl object)
// to add the user control in your master page