Thanks for writing and for your question.
The easiest way for you to download the source code would be to download the Telerik Control Panel and use its options.
You can also bypass this route by going to your Account Overview (by clicking on your Name on the top right edge of our website), then clicking on Manage Products on the menu above your account profile picture and clicking on RadControls for Windows Phone. Then you will see a blue button which says "Download Installer and other resources".
I hope this helps.
All the best,
the Telerik team