If the Target property of the RadButton (ButtonType="LinkButton") is set to a custom string (the name of the target window, where the content of the linked page will be rendered), the link is not opened. For the time being the following workaround can be used:

JavaScript:

    <script>
        function OnClientClicked(sender, args) {
            var target = sender.get_target();
            var navURL = sender.get_navigateUrl();
            window.open(navURL, target)
        }
    </script>

ASPX:

<telerik:RadButton ID="RadButton1" runat="server" ButtonType="LinkButton" Text="click" Target="MyWindow" NavigateUrl="http://www.google.com" OnClientClicked="OnClientClicked" />