Ok I've found out for myself so I'll share it with you lot 
Here goes.....
Open the USBAPI.h file located at Java/hardware/arduino/sam/cores/arduino/USB/....
Look through the list and you will find the following section:
//===============================
//===================================
// Keyboard
#define KEY_LEFT_CTRL 0x80
#define KEY_LEFT_SHIFT 0x81
#define KEY_LEFT_ALT 0x82
#define KEY_LEFT_GUI 0x83
#define KEY_RIGHT_CTRL 0x84
#define KEY_RIGHT_SHIFT 0x85
#define KEY_RIGHT_ALT 0x86
#define KEY_RIGHT_GUI 0x87
#define KEY_UP_ARROW 0xDA
#define KEY_DOWN_ARROW 0xD9
#define KEY_LEFT_ARROW 0xD8
#define KEY_RIGHT_ARROW 0xD7
#define KEY_BACKSPACE 0xB2
#define KEY_TAB 0xB3
#define KEY_RETURN 0xB0
#define KEY_ESC 0xB1
#define KEY_INSERT 0xD1
#define KEY_DELETE 0xD4
#define KEY_PAGE_UP 0xD3
#define KEY_PAGE_DOWN 0xD6
#define KEY_HOME 0xD2
#define KEY_END 0xD5
#define KEY_CAPS_LOCK 0xC1
#define KEY_F1 0xC2
#define KEY_F2 0xC3
#define KEY_F3 0xC4
#define KEY_F4 0xC5
#define KEY_F5 0xC6
#define KEY_F6 0xC7
#define KEY_F7 0xC8
#define KEY_F8 0xC9
#define KEY_F9 0xCA
#define KEY_F10 0xCB
#define KEY_F11 0xCC
#define KEY_F12 0xCD
You can now add your own which you can add from this list as follows:
// missing function keys
//
#define KEY_F13 0x68
#define KEY_F14 0x69
#define KEY_F15 0x6A
#define KEY_F16 0x6B
#define KEY_F17 0x6C
#define KEY_F18 0x6D
#define KEY_F19 0x6E
#define KEY_F20 0x6F
#define KEY_F21 0x70
#define KEY_F22 0x71
#define KEY_F23 0x72
#define KEY_F24 0x73
#define KEY_F24 0x73
// missing sound control keys
//
#define KEY_MUTE 0x7F
#define KEY_VOLUMEUP 0x80
#define KEY_VOLUMEDOWN 0x81
// Numeric keypad
//
#define KEY_KEYPAD_0 0x62
#define KEY_KEYPAD_1 0x59
#define KEY_KEYPAD_2 0x5A
#define KEY_KEYPAD_3 0x5B
#define KEY_KEYPAD_4 0x5C
#define KEY_KEYPAD_5 0x5D
#define KEY_KEYPAD_6 0x5E
#define KEY_KEYPAD_7 0x5F
#define KEY_KEYPAD_8 0x60
#define KEY_KEYPAD_9 0x61
#define KEY_KEYPAD_DECIMAL 0x63
#define KEY_KEYPAD_ENTER 0x58
#define KEY_KEYPAD_PLUS 0x57
#define KEY_KEYPAD_MINUS 0x56
#define KEY_KEYPAD_MULTIPLY 0x55
#define KEY_KEYPAD_DIVIDE 0x54
#define KEY_KEYPAD_NUMLOCK 0x53
#define KEY_KEYPAD_EQUALS 0x67
Will update further but the volume ones are a nice simple way of controlling the computers volume!