Really sorry for misleading you with our previous reply. Indeed you are right -- the marked zones feature is not available in the Silverlight3 version of RadChart so if you want to use it, you will need to upgrade your project to Silverlight4 and the latest version of the chart binaries for Silverlight4.
We apologize for not resolving this confusion earlier.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items