Lin will ask the character to give her anal sex in her laptop scene where she is viewing the porn. I think the optiion either comes available then or shortly after as she will ask for more anal (at least the little buttslut did in mine) Haven't got to the contest yet and she has had anal sex 25 times already. Everything stat wise on her is at least 20 except handjobs and footjobs. Hand jobs I stopped at 12 as she was already a pro and if I recall hanjob isn't a big deal with the contest. Her slutiness is well over 100 already. Just keep at it as a lot of stuff just randomly pops on its own to progress the story. Or at least that's how it seems anyway. I ignored almost everyone else and just concentrated on building up Lin from the getgo. Kali is the only one I have finished all levels of dungeon with other than Lin and Jin so far. Started training the little bitch Ash in there recently but not far with her yet.