You need to run MultiBoxClient on both machines.

1) Run MultiBoxServer on the machine with the keyboard and mouse (machine A).
2) Run MultiBoxClient on machine A.
3) Run MultiBoxClient on any other machines.

Before running MultiBoxClient on any computers other then machine A make sure MultiBoxClient successfully connected on machine A, if it doesn't then you will lose your keyboard/mouse.

If you do lose your keyboard/mouse you can press escape 8 times in a row to shutdown MultiBoxServer remotely.