Web Forms :: Activate ChecKBox Validation When Clicking Button In Created User Wizard Control?

I have a checkbox on my page that must be ticked and to validate it I am using a custom method as below. The problem is that I now put the checkbox and method within a Create User Wizard custom tempalte and when I submit the form it does not activate the validation check. If I put a normal button on the page outside the tempalte it works. Not sure If I a missing something. Perhaps I can activate it in Created User Event handler in code behind. All Button in my create user Event.

<asp:Button
ID="StepNextButton"
runat="server"
CommandName="MoveNext"
OnClientClick="ShowMP();"
align="middle"
CausesValidation="True"
Text="Submit
my Profile"
/>

aspx
<asp:CheckBox ID="Terms" runat="server" />
<asp:CustomValidator ID="valTandCs" ClientValidationFunction="ValidateTandCs" runat="server" ErrorMessage="Please accept Terms and Conditions before submitting."></asp:CustomValidator>
Javascript
<script language="javascript" type="text/javascript">
function ValidateTandCs(source, args)
{
args.IsValid = document.getElementById('<%= Terms.ClientID %>').checked;
}
</script>