There is actually a very easy solution to this that I found. I traced the error back to the AjaxControlToolkit (ACTK). If you open the ACTK solution and go to the TK project within the solution, just add a reference to Microsoft.Web.UI.Design. Then, just recompile and you are finished. If you try to compile the ACTK without first doing this, you will see the error. Once I did this and referenced the new AjaxControlToolkit.dll, everything was fine.