@Toybox:
Just launching the app gave you that error? If so, what version of Mac OS X are you running? I've not seen that.
Or did it happen when you clicked the "Play" button? If so, do you, by any chance, already have Dominator in your Custom Mansions folder? On my machine, this causes the error you're seeing when the "Play" button is clicked. Move BOTH Chase the Bots AND Dominator out of your Custom Mansions folder and it should work. (Decorum has a copy of the mansions inside its application bundle, and will move them into your Custom Mansions folder when you press "Play". If they're already there, you get an error.)
As for the rest...well...yeah, you do have to learn how to set it up, and I recognize that this isn't ideal. The whole thing is geared towards mansion developers packaging their mansions inside Decorum and distributing them that way, so the process is not as user-friendly as it would be if I expected "normal" users to do it. The program itself is dead simple to use, but there's that barrier to entry of learning to set it up.
What follows is a long account of how Decorum came to be and why it is as it is. Read on if you're curious.
Decorum actually began life as "Custom Asset Enabler", a program designed to enable developers of scenarios for Ambrosia Software's SketchFighter 4000 Alpha to use their own graphics. I wrote it then because SketchFighter's data files live inside the application's bundle, and the procedure for swapping them out is considerably more involved than it is for Midnight mansion. In fact, it's so onerous that it would be completely unreasonable to expect people to do it.
Unlike with MM, SketchFighter maps with custom assets would be pretty much untenable without a utility that performed the swapping for you, every time you wanted to play. This is where the developer-oriented mindset comes from; I knew that, in that case, mapmakers would need the utility to even consider making maps with custom graphics.
When I began beta testing Dominator for Joeb, I found the file swapping to be quite a headache. I soon realized that I had already solved the problem when I created the Custom Asset Enabler. The program's code only required a few simple changes in order to work with Midnight Mansion. And so Decorum was born.
However, the user/developer landscape was quite different for Midnight Mansion. Users were already accustomed to swapping files, and the barrier was low enough that many developers were requiring it. I guess people were confused by that configuration step that was required of normal users because no developers decided to distribute their mansions in a Decorum wrapper.
If I knew then what I know now, and if I had written Decorum from scratch for MM, I would have done things differently. I would have had a drop well in the GUI for users to drop mansion files and associated data into; this would build a library of mansions with custom graphics that could be swapped in by selecting them from a list and pressing a button.
Of course, that would also have been a lot more work, and the program may never have existed at all had I been that ambitious about it. It certainly will never happen now that its reason for existing has been almost entirely obviated by MM 1.0.8.
In its current form, Decorum provides a rather nice solution for those old mansions that have yet to make the jump to the 1.0.8 format (and for Dominator, which can't). It does just want I want it to do, and I'm satisfied with the way it works. So, when people complain about having to swap files and what a pain it is, I can't help but point out that it's a solved problem, at least in my eyes.
If you have any questions about the set-up process I'd be happy to answer them. It's really not that bad once you know how to do it.