VISA notification letter from china

my name is stephen awanovwe, I am the md of interlock electronic systems ltd in abuja, and i was invited to the 4 china international sourcing conference and show on public safety and security in guangzhou from 22-27 july 2011. I have my invitation letter and hotel reservation and other document but your staff at the embassy would not giver me appointment they ask me for notification letter from china, but i learn that the letter is now void what do i do i wish to attend the even please kindly assist me thanks.