For anyone who's interested, here's the procedure I used:
In Thingworx Studio:
1) Open an experience that you want to switch to model-based tracking
2) Remove any existing thingmark in the experience, and set "thingmark association" to "none" in Experiences window
3) In the 3D Home screen, add a modelTarget widget to your model
4) In "Model Target Details" on the right, there will be a rectangle labeled "Detect Position", initially with a gray X in it. Click there.
5) Set the angle from which the user will scan the object to initiate tracking (usually the front), and click Save
6) Save and Publish the experience
In your Thingworx View app:
1) Open the app, tap the library icon (like a stack of cards)
2) Switch to the library view (icon looks like books on a shelf)
3) Find the experience with the title matching what you just published.
a) In iOS, I could just tap on that title directly to start the experience
b) In Android, I had to create a bookmark first, and then switch to the bookmarks view and select it there
4) You should now see a white outline of your model. Point the camera at the real object so that it matches the white outline as closely as possible. This will take a bit of adjusting, both distance and angle, to get it right. Once you get the right angle and distance for a good match, you'll need to hold it steady for a few seconds. When it acquires the tracking, your virtual model should appear in the experience.
5) From here you should be in good shape, ready to play any sequences or any other kinds of interactions you've defined for your experience. You should be able to move around the physical object (keeping it in the camera view) and have the experience maintain the tracking on it.
