a few notes:
in TD
when you save, a file will be created in "assettocorsa\apps\lua\TrackDecoration\dataSections" named "some_track.ini" (whatever track you are editing)
you have 2 options at this point. 1 is original way (but more pain for many objs), 2 is the easy way
1. copy/append the contents of that ini to the existing models.ini in your track folder and add all the kn5 from the ./obj folder to the track folder (i told you its a pain, right?)
a. if you have no models.ini or similar, create one from the TD ini or another track as a template
2. after saving, click "make kn5", there is now a new "some_track.kn5" in the "dataSections" . Since it is likely the same name as the track, rename it to something like "some_track_objs.kn5". Copy this to your track folder. Add the
name of this kn5 to the tracks models.ini (you may need to create one if none exists)
The TD page has more info as well
hope this helps.