use AjaxControlToolKit Config
options:
1.
.aspx file:
<%@ Register assembly="AjaxControlToolkit" namespace="AjaxControlToolkit" tagprefix="asp" %>
...
<asp:Accordion>...
2.
web.config file:
...
<system.web>
<compilation debug="true" targetFramework="4.0">
<assemblies>
...
<add assembly="AjaxControlToolkit" />
...
</assemblies>
</compilation>
<pages>
<namespaces>
...
</namespaces>
<controls>
<add tagPrefix="asp" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" />
</controls>
</pages>
</system.web>
...
.aspx file:
<asp:Accordion>...