Koffin ML --- Go to Outworld (Location C 3) and fight her.

Im having the same problem. Tanya isnt there to fight and get her alt. >

first you have to find her in the south i think, talk to her, then she will say: find a cave where I can trap liu kang. you then go behind Baraka's villiage
walk in the cave, it'll tell you if its the right one, then go back and tell Tanya.
Leave outworld, then come back in and Tanya will be in c3 or whatever.

Yea, you'll find her in A-7. She'll ask you to help her find a cave to trap Liu Kang. Just go to G-4 (Baraka's camp) and follow it around to the back and then a cut scene will happen and you will get the "Map to Cave" in your inventory. Just take it back to Tanya in A-7 and then leave Outworld then reenter. Just follow the bridge (the straight one, not the one on the right) from the portal and she'll be right there. Just fight her.

i didnt go to a cave i went to find liu kang in that place that hotaru owns and he said "thanks for warning me"
and i went to see tanya and she wasnt there so then i found her where ever it is you find her after the mission and killed her and got her costume.
so i guess u can do both

mkd fan has right ... i gave the map cave to tanya ... i'm gooing to nexus , I go back to tanya on the bridge where te portal , i beat her, i recieved the coins exept her alt. costume ...

You have to do her mission to trap Lui Kang first. The cave you have to find to trap him in is in a locked area I think so you wait until the second time you enter Outworld. Do her mission when you re-enter Outworld by finding the cave in the now unlocked area. Leave outworld. The next time you are required to enter Outworld she will be there at C-3.