I'm thinking that when you say "Seduce Madison or Amy" you mean going down their forced path with the "rape" button. You can do that to Amy, but only if the 2 of you are alone. If there are people around, you'll get beat up, so make sure there are no witnesses. Once Amy's "forced" path is complete, you can repeat the sex with her again with the same conditions, no witnesses. With Madison, it will let you force her, but only if you're alone and as soon as your done, she'll call for frank. Amy won't call for help afterwards.
As far as the limited characters, talk to Derek. As soon as you do, it opens up ability to talk to Madison. Pay attention to the pop-ups on the right side of the screen. As you talk to people or accomplish tasks they need you to, other characters will unlock and it will display as a pop-up on the right side of screen.
The only 2 paths that are complete and functional right now are Amy and Madison's "forced path". Most of the other girls like Rachael and Katherine work the same as with the original story, with some minor tweaks that require additional conditions to be met to sleep with them. I'm working on Madison's blackmail path, and it should be up and running in the update tomorrow or the day after so watch the forum for the updates. After that, updates will be released that complete the seduction paths and the drug paths. And finally, a final update with the alternate ending cinematics, depending on which path you chose for each girl and the status of the completion of Derek's payment quest.
Any other questions, feel free to pm me.