Thanks SlidHydra647449. I’m pretty sure the polygon closing procedure at the moment is CTRL+Double left-click. Happy to be proved wrong though. My issue is that I can’t place any points at all, although the double-click to finish does seem to be regsitered as such by the SDK.
For anyone coming to this later on the solution is easy.
Due to some SDK bug the polygon draw is dis-functional for some of us;
Workaround for now;
After you’ve clicked for your first point for the polygon, drag the mouse over to the Scenery Editor window and click on the top of it to turn the Scenery Editor window from black back to blue.
Rinse and repeat for each point of your polygon. Double click to finish.
Voila! your Polygon is complete and you can assign the attributes in the usual way.