Your code should seem something like this. The content you need to display should be mentioned in the PopupControlID.
<asp:popupcontrolextender id="PopupControlExtender1" runat="server" xmlns:asp="#unknown">
TargetControlID="Button1"
PopupControlID="Panel1"
Position="Right"
CommitProperty="value"
OffsetX="3">
</asp:popupcontrolextender>
<asp:panel id="Panel1" runat="server" xmlns:asp="#unknown">
<asp:label id="lblInfo" text="Enter shipping information and the order can be completed" runat="server"></asp:label>
</asp:panel>