You gotta wait until a certain date. I am not sure what date it is, but there are guides everywhere that tells you how. See the Krypt threads above! Or, check Bradygames.com

To Unlock Him you must complete Konquest Mode. Once you complete Konquest mode the key to his grave appears back in the Konquest Mode. You must go thru the Konquest Mode again to get the key.
He is in Koffin OT. The key to his Koffin is in Edenia section G8, behind the tent. It only appears at 12:00 AM Friday until 12:30 AM, that 30 seconds.

I have tryied to unlock him but im done with konquest and i did the rain mission and that Jade i think it is was first at the enter of the cave standing next to rains body and then i go in to find liu kang thingy and there is jade agian in the tent and i cant find the thing behide the tent i think its Jade that mess it up its the same with tanja i cant find her she went missing after she asked me about finding a cave when i got back to her she was missing !
So i cant unlock them i guess i need to start over !
PS: Getting scorpion ́s move by beating him is so hard !
Plus beating kobra so awake something !
And i now its not nice to make new things to talk about in a thread that has one goal but i think to get SU you will get a kode when the gamecube is out !
I got xbox version by the way

Awesome, thanks!
Now I have Liu Kang ^_^

I got Liu Kang by doing a mission with Rain in which he sends you to go look for Jade and then Jade tells you to go back to Rain and send him to her and then when you go back to Rain and then back to Jade he is there, dead on the floor. Jade says something like "Purple? Rain has such weird taste in colors" and then at a certain time u have to go behind the tent and get it. It's complicated but it says it all in Krypt pages and all those cheat thingies.