Ab Mukhtar Haqbeen,
Just try to answer your question about mental object. The mental object is any object that we imagine in our mind, so we create a mental image in our mind about certain object. In the case of meditation, the mental object suggested for us to imagine is something round and easy, like the moon, the sun or a round bubble. Then you need to imagine to put that object in the center of the body, which is around 2 cm above your navel (in your stomach). If it is hard for you to imagine it in the stomach right away, the clip video suggested to learn to imagine it first to put the object that you imagine (the moon for example) in front of you, then you can move it on your shoulder, then on top of your head, and then you can imagine to swallow that object going down to your throut, go down to stay in your stomach. There you feel the object stay in your stomach, which is around the center of the body. So all you need is you imagine/create in your mind about the object. But actually the purpose is not in creating of the object itself, but simply help us to concentrate and put our mind still in the center of the body. So creating the mental image is not the end purpose, it is only our way or technique to help us still our mind in the center of the body.
What I have learnt from my peace coach is that actually we do not need to worry so much about the mental object and where to place it so that we loose the "sabai" peaceful feeling that we are supposed to enjoy in the meditation. I guess we just need to keep on practicing it so that we would learn it better.
Well, that's all I can share about it.. I am learning it too...
Keep it up, let's learn together...
Maybe other friends too can help ... :)
peace, berta