Is very common to need to access a Silverlight element programmatically. There are numerous ways to do so. Assuming you have named your element as shown here
<Rectangle Name="myRect" Width="100" Height="44" Canvas.Left="0" Canvas.Top="10" StrokeThickness="2" Stroke="Black" />
One very simple way to access this rectangle from the handleLoad function is by using getItem() and indexing into the children collection of the root element
this.rect = rootElement.children.getItem(0); this.rect.strokeThickness="5";
An alternative and more general approach is to use findName(), which you can also call on any element, including the root element
this.rect2 = rootElement.FindName("myRect"); this.rect2.Fill = "Red";
Note this quote from the documentation: the object you are searching for does not have to be a direct descendant of the object that invokes the findName method.
In addition, you can call FindName() on the plugin's content sub-object
var rect3 = plugIn.content.FindName("myRect");
rect3["Canvas.Left"] = 300;