スイッチをBS(バックスペース)キーにする-USB Device - HID - Keyboard-PIC18F2550

キーコードを調べます。
USB HID toPS/2 Scan Code Translation Table
によると、バックスペースは、2A　のようです。

void Keyboard(void)
{
 static unsigned char key = 4; 

 //Check if the IN endpoint is not busy, and if it isn't check if we want to send 
 //keystroke data to the host.
    if(!HIDTxHandleBusy(lastINTransmission))
    {
        if(Switch3IsPressed())
        {
         //Load the HID buffer
         hid_report_in[0] = 0;
         hid_report_in[1] = 0;
         hid_report_in[2] = key++;
         hid_report_in[3] = 0;
         hid_report_in[4] = 0;
         hid_report_in[5] = 0;
         hid_report_in[6] = 0;
         hid_report_in[7] = 0;
            //Send the 8 byte packet over USB to the host.
            lastINTransmission = HIDTxPacket(HID_EP, (BYTE*)hid_report_in, 0x08);
 

とりあえずkey++の場所に0x2Aと記入すれば動きます。