Hi Telerik,
I tried to find the text from combobox doesn't work using the normal dropdownlist javascript as below

<telerik:RadScriptBlock ID="rsbAction" runat="server">

    <script type="text/javascript">
        function action(sender, args) {
            var rcbCategory = document.getElementById('<%=rcbCategory.ClientID %>');
            var result = false;

            if (rcbCategory != null)
                if (rcbCategory.selectedIndex > 0)
                    result = true;
            args.IsValid = result;
        }

    </script>

</telerik:RadScriptBlock>
I as well tried the code provided by the sample also doesn't work as below :

<telerik:RadScriptBlock ID="RadScriptBlock1" runat="server">
    <script type="text/javascript">
        function category(sender, args) {
            var result = false;
            var rcbCategory = $find("<%=rcbCategory.ClientID %>");
            var rcbCategoryText = rcbCategory.get_text();

            if (rcbCategoryText.Length > 1) {
                var node = rcbCategory.findItemByText(rcbCategoryText);
                if (node) {
                    var value = node.get_value();
                    if (value.length > 0 && value % 2 == 0)
                        result = true;
                }
            }
            args.IsValid = result;
        }
    </script>

</telerik:RadScriptBlock>

Thank you.