Okay, this was posted on the GameFaqs forum and I'm currently doing it to see if it's 100% accurate, but until then, I'm posting it anyways, for everyone who has the game and is deep in Konquest mode. You can ONLY search for Liu Kang and Raiden once you have beaten Konquest mode all the way through and received Shujinko as playable in Arcade mode. Then, once you go back to find items you missed the first time around can you unlock Liu Kang. and Raiden. And I warn you, to unlock Liu, it takes WORK.
~Raiden~
Go back to Orderrealm and fight him again if you already did before you beat the game. Once you beat him, he will give you the key to the krypt in which you unlock him.
~Liu Kang~
You *must* complete Part 1 to open Part 2.
-Part 1
1) Return to Earthrealm
a) Train with Liu Kang beyond the bridge in which you see Jax
b) Defeat Kung Lao at the Shaolin Temple
2) Go to Netherrealm
a) Unlock the gate in the area in which you could not enter before beating Konquest mode by defeating Kabal, Kira, and Kobra in kombat
b) Make sure you have done Shinnoks quest (you need his favor later)
3) Head to Edenia
a) Search for the following characters; Sub-Zero, Scorpion, Havik and complete their challenges
b) Meet Queen Sindel who will tell you that Kitana has been made into the Dragon Kings bodyguard, and only her love of Liu can free her. But you have to unite Liu's Spirit and Body.
This all must be done before moving on
-Part 2
1) Head back to Earthrealm
a) Talk to Spirit Liu and defeat his challenge (you bleed, he does 300% damage/healing)
2) Head to Edenia
a) There is a tent in the corner of the map, Liu's spirit and Sindel are there. Talk to both. Beat Sindel's challenge (darkness, bleeding)
3) Finally head to Outworld
a) Talk to Liu (Living Liu) in the city you gave to Order. He will tell you only Shinnok has the power to free spirits.
b) Return to Shinnok (probably in Netherealm) to reunite Body and Spirit. When you return to Edenia you will have to fight Shao Khan, who will then allow you to unlock Liu Kang.