From what I can tell, that entire code snippit is for the older WinForms control, even though the help topic is for the new WPF map and other controls. Unfortunately I've seen MANY such inconsistencies in the version 10 help.
In any case, I believe what you are looking for is:
mapControl1.MapLayers.AddRange(mobileCache);
If you look at the "Walkthrough_WPF" sample under "C:\Program Files\ArcGIS\Mobile10.0\DeveloperKit\Samples\MobileApplications" this might help you. I am unable to get that sample to run at the moment, but that's another story/post!!!