Hello,
Please look for the web.config file in the root of your project folder. This is general one, used for the whole application.
There should be following structure:
...
<system.web>
<pages validateRequest="false">
<controls>
<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
<add tagPrefix="ajaxToolkit" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" />
<add tagPrefix="cms" namespace="CMS.Controls" assembly="CMS.Controls" />
<add tagPrefix="cms" namespace="CMS.FormControls" assembly="CMS.FormControls" />
<add tagPrefix="cms" namespace="CMS.ExtendedControls" assembly="CMS.ExtendedControls" />
<add tagPrefix="cms" namespace="CMS.PortalControls" assembly="CMS.PortalControls" />
</controls>
...
Basically the following section is the one you should add it to:
<system.web>
<pages validateRequest="false">
<controls>Hope it helps.
Best regards
Ondrej Vasil