Ajax modalpopupextender not popping up - asp.net

Question

.modalBackground
{
background-color: Gray;
filter: alpha(opacity=70);
opacity: 0.7;
}
.modalPopup
{
background-color: #ddffdd;
border-width: 3px;
border-style: solid;
border-color: Gray;
margin-top: 60px;
padding: 2px;
width: 400px;
font-size: 10pt;
}
<cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server" CancelControlID="Button3"
OkControlID="btnOk" TargetControlID="LinkButtonDummy" PopupControlID="PanelPopUp"
BackgroundCssClass="modalBackground" />
<asp:Panel ID="PanelPopUp" runat="server" CssClass="modalPopup" Style="display: none">
<div>
<asp:Label ID="lblMsg" runat="server" />
<asp:Button ID="Button2" runat="server" Text="Add New Organisation" OnClick="Button2_Click" />
<asp:Button ID="Button3" runat="server" Text="Cancel" />
</div>
</asp:Panel>
I have also included the AJAX reference and a ScriptManager on the master page.
I have to add an enhancement to an existing page and I'm quite new with ASP.NET, the page in question is a 'content page' and is linked to a master page (containing the scriptmanager).
This code all looks completely fine and I have been reading on this for over three hours now but to no avail - my modal doesn't 'pop-up' and grey the background out, it simply appears where I have placed it on the page (right at the top, or right at the bottom e.t.c.) as if I was just showing/hiding a div.
Can anyone help, I'm going crazy?
What other routes can I go down for adding a confirmation box on a page that takes a string built in the code-behind and also runs code-behind functions on OK/Cancel?

Answer 1

Keep your css as it it and try this code :
<asp:HiddenField ID="HiddenField1" runat="server" />
<asp:ModalPopupExtender ID="MyPopup" runat="server" CancelControlID="Button3" OkControlID="btnOk" PopupControlID="PanelPopUp" BackgroundCssClass="modalBackground" DynamicServicePath="" Enabled="True"
TargetControlID="HiddenField1"></asp:ModalPopupExtender>
and on Button2_Click event add
MyPopup.Show();