Go to whatever visual studio version you are using and open the visual studio command prompt in admin mode (right click the shortcut, run as administrator...the command prompt might be named something slightly different depending on what version of VS you are using).
Then copy / paste this into the command prompt (changing this slightly for whatever version of the .net framework telerik version your have installed, if you have multiple versions installed do this for each one). Also note depending on what version of the Telerik controls you have installed, you might have to change the "UI for ASP.Net...." path to whatever path your telerik bin lies in.
gacutil /i "C:\Program Files (x86)\Telerik\UI for ASP.NET AJAX Q1 2014\Bin40\Telerik.Web.Design.dll
I hope this helps someone, credit goes to my boss for finding this.