Need help with USBIN

Hi,

I am making a PS2-USB keyboard converter with an 18F4550, which will allow me to convert any ps2 keyboard to a hybrid qwerty/dvorak USB keyboard.

I've got the ps2 keyboard -> pic communication working.
I've got the pic -> pc(via usb) working.

What i havn't been able to wrap my head around is the PC -> pic communication (used for sending the led status reports). I'm unsure as how USBIN works.

I have attached the HID descriptor file i'm using.

Does anyone have any idea how the PC->pic communication works?