PLANET KVM-E500 - Manual (Page 8)

6. The Mouse is not detected during PC boot up.
6.1 Make sure the mouse works when directly plugged into the
computer.
6.2 Make sure the mouse is a true PS/2 mouse. A combo mouse
will work just as long as it is set for PS/2 mode with the correct
adapter.
6.3 Try a different mouse.