Selecting an item from the RadComboBox and then pressing Enter key, does not trigger the default button(if set) of the form/panel in IE and Chrome. The workaround is to handle the client-side OnClientKeyPressing event of the RadComboBox and manually trigger the clicking of the default button.For example:

ASPX:

    <asp:Panel ID="Panel1" runat="server" DefaultButton="RadButton1">
        <telerik:RadTextBox ID="RadTextBox1" runat="server">
        </telerik:RadTextBox>
        <asp:Label ID="Label1" Text="" runat="server" />
        <telerik:RadButton ID="RadButton1" runat="server" OnClick="OnClick1" Text="Click" />
        <telerik:RadComboBox ID="RadComboBox1" runat="server" OnClientKeyPressing="OnClientKeyPressing">
            <Items>
                <telerik:RadComboBoxItem Text="text1" Value="val1" />
                <telerik:RadComboBoxItem Text="text2" Value="val2" />
                <telerik:RadComboBoxItem Text="text3" Value="val3" />
            </Items>
        </telerik:RadComboBox>
    </asp:Panel>

JavaScript:
    <telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
        <script type="text/javascript">
            function OnClientKeyPressing(sender, args) {

                if (args.get_domEvent().keyCode == 13) {
                    $find("<%=RadButton1.ClientID %>").click();
                }
            }
        </script>
    </telerik:RadCodeBlock>