Are you not being able to activate the lock switch or are you seeing the focus point moving from where you set it? Be sure it is not on Auto Area mode, which will move the AF point to where it thinks the closest target, unless you intend the camera to take control. The Focal point lock level switch is turned clockwise to the "L" mark, the multi selector becomes disabled from controlling the focus point. Stan St Petersburg Russia
Let my try to help. I too have a new d7100 and like to try things people are asking so I can learn more of the cameras potential.
Pg 3 of manual #12 button. Press it and then turn dial sub dial pg 4 , # 2, and you will get a view like p74 "viewfinder" upper right or a single point. The single point can now be moved using multi selector, p5, #14. Set focus selector to dot and use multi dial to move focus point. Then switch focus selector to L to lock it anywhere you want. I hope I have explained what I was doing to try and answer your question.
The manual doesn't make this as obvious as it should be. Here is what you need to do:
Move the lock lever on the multi-selector from "L" to "dot"
With the camera on, hold down the AF button (just to left of lens) and spin the rear dial to select your AF mode (AF-A, AF-S, AF-C) I use AF-A or AF-S for shooting stationary objects
While holding down the AF button, spin the front command dial to select AF mode. Choices are AUTO, 3D, 51-point, 21-point, 9-point, single point. In any mode other than AUTO, you can move the central focus point with the multi-selector.
To quickly move the focus point to the central point, press the OK button on the multi-selctor.
If desired, lock the position of the focus point by turning the level on the multi-selector back to "L"
Hope this helps. The number of choices was a bit mystifying compared to my previous camera, a D100, which had a whopping 5 (!) focus points and only two modes, AF-A and single point.