Your FilteredTextBoxExtender will only allow you to enter alphabets. In order for you to allow it to enter special characters you will have to set the filter type to custom and change the code as follows.
<table><tbody><tr><td class="auto-style9" colspan="3" style="vertical-align: top;">
<asp:textbox id="txtmilestone_name" runat="server" width="408px" xmlns:asp="#unknown"></asp:textbox>
<cc1:filteredtextboxextender id="FilteredTextBoxExtender1" runat="server" filtertype="LowercaseLetters, UppercaseLetters, Custom" validchars="!~@-" targetcontrolid="txtmilestone_name" xmlns:cc1="#unknown" />
<asp:requiredfieldvalidator id="RequiredFieldValidator1" runat="server" controltovalidate="txtmilestone_name" errormessage="Milestone name field required!" forecolor="Red" validationgroup="m_add_validation" xmlns:asp="#unknown">*</asp:requiredfieldvalidator>
</td></tr></tbody></table>