Unfortunately there is no shared memory in Silverlight, at least not in standard terms. You only have local messaging, which is just sending strings from one application to another.
The blog post you link to refers to "Shared memory" as an implementation detail of Local Messaging, i.e. when you pass a string from one app to another it will not actually be copied. This though does not allow us to share memory in a more usable way. The benefit of having the strings shared may be outweighed by serializing the objects and making sure that they exist only in their serialized (string) form.
Here is a blog post about using the local messaging (you may have seen it already):
the Telerik team