***Spoilers***

So the Demon's just arrived and all the walkthroughs say to run into the secret passage except it isn't open.. Is there something I should have done to get it to stay open?

I started a new game in Chapter 7 as in my main game I thought to myself "cool! I finally get to see what's in the dark room and walked right in.." then it wouldn't let me return to my save.

I'm really frustrated with this now so any help will be greatly appreciated.

You *should* have the secret passage opened if you start Chapter 7 when you make a new game. It's kinda one of the things that make chapter 5 complete.
You might have come across a bug, though. See if reinstalling the game helps.

Running into the darkroom as first thing in Chapter 7 is not a problem either, as long as you try to escape Don all the time. ;)