web.config - make sure the version matches your MySqlClient version

  <system.data>
        <DbProviderFactories>
            <add name="MySQL Data Provider" invariant="MySql.Data.MySqlClient" description=".Net Framework Data Provider for MySQL" type="MySql.Data.MySqlClient.MySqlClientFactory, MySql.Data, Version=5.2.7.0, Culture=neutral, PublicKeyToken=c5687fc88969c44d" />
        </DbProviderFactories>
    </system.data>

SqlDataSource reference

<asp:SqlDataSource ProviderName="MySql.Data.MySqlClient" ID="dsData" runat="server" ></asp:SqlDataSource>