yup, that method has been renamed from Expanded to IsExpanded. Since you're running in Rhino5 Grasshopper is using the new SDK. This error will not happen in Rhino4 and it will go away once the next Grasshopper version is released.
In the meantime there's nothing* you can do, sorry.
--
David Rutten
david@mcneel.com
Seattle, WA
* Short of using Rhino4 or not clicking on the Bake menu item....