I’m playing around the Relate+ demo site and would like to know how to add ContentArea to the start page of the demo site (my intention was to add blocks to this ContentArea)
It's actually pretty easy. As Relate+ sample site does not create page types from code, all you need to do is to add property directly to the page via UI:
Then you need to add property to template markup anywhere you want to content area to show up:
<EPiServer:Property ID="epiSampleArea" PropertyName="SampleArea" runat="server" DisplayMissingMessage="false" />
Then you can see content area in edit mode:
Actually what I would do is to use excellent plugin made by Alf (http://talk.alfnilsson.se/2013/03/08/creating-page-type-classes-from-page-types-in-episerver-cms-7/) to reverse engineering page types back to code and add ordinary property to the page type class.