It is my understanding that most newer USB keyboards will not work with the PS2Keyboard library because they do not use the PS/2 protocol. Some, but not all, do have the ability to use the PS/2 protocol:
Many keyboards and mice can connect via either USB or PS/2, selecting the appropriate protocol at power-on. Such devices are generally equipped with a USB connector, and ship with a passive connector adapter to allow connection to a PS/2 port. Conversely, modern USB keyboards and mice usually have only USB plugs and require an active adapter to connect to a PS/2 port.
If it has the capability, would a USB keyboard select the appropriate protocol when wired to an Arduino automatically? If not, is there a way I can force it to select PS/2?
If the USB keyboard does not have the protocol selection capability, would an active adapter work with an Arduino?