I assume you have a control.
private void jprad_MouseEnter(object sender, MouseEventArgs e)
{
ToolTip sample = new ToolTip();
sample.Content = "JAPAN";
jp_rad.Content = sample; //Japan Radio Button
}
we have added our Tooltip object in MouseEnter of a radiobutton named jprad which has a value of "Japan"
by assigning Content property of any control,you can show tooltip in Silverlight dynamically.
Cheers,