The phone holder can be a bit challenging to install, especially the first time (before it has been broken in a bit). You need to rock the holder onto the base unit, starting from the side that has the smartphone holder itself. It won't install from the other direction, nor will it click in place if you press straight down.
Follow the steps and watch the video below:
1. Align the cutout in the smartphone holder to the boss on the base unit, rocking from the side that has the "surf board" and smartphone holder itself.
2. Pull the other side of the holder up and over the boss on the other side of the base unit, applying upward pressure on the lip. You'll hear an audible click as it moves into place, and it takes a bit more force than you'd expect.
3. To remove the holder, begin from the non-smartphone side and unclick it from the boss.