If you've played everything except the hidden ending, you should have all of the bonus content available except for some scenes and a CG in the gallery pages. If you're having trouble finding the "Aftermath" content, go to the title screen and check the bottom-right. If you've finished at least one true ending or the normal ending, there should be a new button there.
As for the hidden route/ending itself, the requirements aren't necessarily intuitive. Here's how you get there:
- You must have seen the true ending for each of the three character routes. (They must appear in the ending list.)
- From a new game, spend exactly one "date" event ("spend time with") with each girl and two events alone. Partner events ("partner with") don't affect this route.
- If you meet these requirements, an alternate infirmary scene will play at the end of Act 2. From here on, you're locked into the hidden route.
Hopefully that helps you find the last ending. Let me know if you have any more questions and I'll try to help.