BusyIndicatornot found in the Silverlight SDK, its in the Toolkit , which you need to download and install.
After installation, add a link to the dll System.Windows.Controls.Toolkit.
BusyIndicator: -
xmlns:toolkit="http://schemas.microsoft.com/winfx/2006/xaml/presentation/toolkit"
toolkit: alias : -
<toolkit:BusyIndicator x:Name="busyIndicator>
<!-- your page content -->
</toolkit:BusyIndicator>