You have to unzip the file "ETS_ATSPlugins.zip" (This .zip is inside the folder "GamePlugins" inside Tom Connector) inside the root directory of the game, if it is in Steam it will usually be in "\steamapps\common\American Truck Simulator".
You will now notice that every time ETS2 starts, it indicates that the SDK has been activated. Unfortunately, you have to hit OK on this message each time, but it is a small price to pay for the additional features that are possible through the SDK.