Changing target framework

I added the AjaxControlToolkit.dll to my project so that I can use some of the added controls it provided. Visual studio said that the dll or one of dependancies needed a higher framework. So I want to project properties->Application and changed the Target Framework to .Net Framework 4. Now it gives me a bunch of errors stating that my controls are not difined. I tried going back to .Net Framework 2 but no luck, it gives me all the same errors. Before the Framwork change everything worked perfectly.