Autocomplete extender not working

Hi Friends,

       Although, i write a code with all required lines in  Asp.net , the Autocomplete extender not working ,i dont able to find the reason anyone can teach with code.

The Error i faced 

Error    3    Could not load file or assembly 'AjaxControlToolkit' or one of its dependencies. The system cannot find the file specified.   
Error    4    Unknown server tag 'asp:ScriptManager'.  
Error    4    Element 'ScriptManager' is not a known element. This can occur if there is a compilation error in the Web site.   
Error    5    Unknown server tag 'asp:AutoCompleteExtender'.   
Error    5    Element 'AutoCompleteExtender' is not a known element. This can occur if there is a compilation error in the Web site.  

AjaxControlToolkit i  installed .