Question How can I make the objects show at the middle of the table?

By default when you author a proxy it will be centered - you can see where it's position will be by looking at the plane visualization in the scene view.

Example:



I'm guessing in this case your content is already is centered and what is happening is the detected plane is merged between the chair and the table. This is a typical device limitation pain point - but also why our device view displays 'imperfect' data - your intuition to have a user adjust the location xz + scale is a good solution.

Adjusting the position/scale/rotation after placement is possible. There isn't anything really MARS-specific about that so we haven't offered an out of the box solution for it - you would select and move it like any other object in Unity. An update of the direct placement template is going to have this functionality as well - I'll ping this thread when it's available.

 

You can find the direct placement template in this thread (it's in active development):
https://forum.unity.com/threads/mars-direct-placement-example.908381/