Assuming your licence covers access to the source code, then you need to:
- Log in to you client account on the telerik site
- Click on "Download and Manage you Products"
- Click on "Download installer and other resources" next to the Ultimate Collection (or Premium Collection if that's what you bought)
- Click on "Browse all product files" for RadControls for ASP.NET AJAX
- The source code is about half way down the page.
Obviously, if you don't have the licence to access the source code, you won't be able to do this, in which case, you need to contact the Sales department