Hi,
you need to add reference to this control like:
<%@ Register Src="~/CMSAdminControls/Debug/QueryLog.ascx" TagName="QueryLog" TagPrefix="cms" %>
Please also don't forget to add these lines into web.config:
<add key="CMSDebugSQLQueries" value="true" />
<add key="CMSDebugSQLQueriesLive" value="true" />
Best Regards,
Zbysek Nemec