Change 'int8' to 'int64' or rather just 'int' (Though I have my doubts on this, I only started memory-reading 2 days ago - AutoIt doesn't seem to care much about declaration)
Returns 0 if you haven't got anything targetted, otherwise the GUID of the mob/player.
You can verify if it works by targetting yourself and comparing to the output of this
Code:
#Include <nomadmemory.au3>
SetPrivilege("SeDebugPrivilege", 1)
Const $Handle = _memoryopen(WinGetProcess("World of Warcraft"))
;Get the pointer to the Object Manager
$g_clientConnection=_memoryread(0x011C8248, $Handle, "ptr")
$s_curMgr=_memoryread(($g_clientConnection)+0x285C, $Handle, "ptr")
;Get the GUID of the player
$playerGUID=_memoryread(($s_curMgr+0x0C0), $Handle, "uint")
Msgbox(0,"",$playerGUID)