Success!
Well sort of... Took me a little while to figure out why nothing was happening.
It appears that telemetry data for plugins not implemented until patch 1.4.1.
Default install of Steam FULL version and DEMO version are NOT the latest patch version.
ONLY steam FULL version can be manually updated to version 1.4.1
(inside steam interface) by right click in game list and choosing properites.
Select BETAS tab
Select Temporary141 opt in from the drop down list.

Steam DEMO version does not have this option. ( currently ver 1.3.1d )
Work around for demo version. Download demo from official site. This version is 1.4.1. ( Non Steam Version ).
http://download.eurotrucksimulator2.com/EuroTruckSimulator2_1_4_1_setup.exeexecutable path - "C:\Program Files (x86)\Euro Truck Simulator 2\bin\win_x86\eurotrucks2.exe"
taskmanager - "eurotrucks2.exe"
windowed mode window title - "Euro Truck Simulator 2"
( I am guessing this is same for full and demo version, because you can turn demo version into full version with just entering a valid license key ?)
After a quick search in the readme for SDK I found...
The game loads all plugins found by combination of following methods:
1) By gathering all libraries from "plugins" subdirectory of the directory containing the game binary (e.g. /bin/win_x86/plugins).
2) (Windows only) By processing all string values stored in registry key "HKEY_LOCAL_MACHINE\SOFTWARE\SCS Software\<GAME_NAME>\Plugins" (GAME_NAME is "Euro Truck Simulator 2" for ETS2). The data of those values are interpreted as paths to the libraries, one path per value. The names of the values are not relevant however it is recommended to use something identifying your product to avoid possible conflicts.
So, I just manually created the "/plugins" folder and put the Truck Plugin dll inside this folder.
"C:\Program Files (x86)\Steam\steamapps\common\Euro Truck Simulator 2\bin\win_x86\plugins\X-Sim.dll"
I selected rFactor plugin as instructed.
Started the game, and when the game started at the character select screen, it brought up a confirmation window to use advanced SDK features.

(SUCCESS!)
When game loads into the game world, data seems to be sent to x-sim and simulator chair responds accordingly.
When you quit game or go into game menu, data stops and chair stops correctly.
That is the extent of my current testing.
At this moment I cannot confirm any accuracy of the forces or which forces are actively being read since I didn't set up any kind of serious profile, just selected speed, gear, pitch to see if any response was activated. So I will continue to fiddle around with this until further advice from you on how to proceed or what else you would like or need for me to do.
Are speed, gear, rpm, roll, pitch the only telemetry data in this v1.0 of truck plugin?
Or, do I need to test all forces on the list and see what forces work?
Thanks!