Sorry guys, but we forgot to include some folders and files into the installation package. That's why you're getting an error when trying to add a custom mod truck to the application.
We'll fix this error with the next release of E2S, but until then - here's a quick fix:
Go to your E2S installation directory, and open the \data\trucks folder.
In there, create a folder named templates .
That should solve your problems, and you should be able to add mod trucks to ETS2 Studio.
Thank you all for reporting the bug, and sorry for any inconvenience.