Unfortunately, if the button has fallen out of your Platform or iKeyboard series device, in order to correctly re-insert the button your hardware will need to be disassembled.
These buttons are designed with a special peg which holds them into the board, though as a result of this peg, the button must be mounted correctly into the board with the unit disassembled.
The options for repairing these buttons are either to clip off the small peg at the end of the button which should allow the button to re-seat into the slot, however this will not hold the button in place, and turning the device upside down will cause the button to fall out.
Alternatively, users can attempt to open the unit and re-insert the buttons themselves, or request service from the distributor to have the buttons replaced.
Here is a link to a list of distributors in certain regions.