MM question:
I believe you should just re-download Midnight mansion at the homepage, and just drag the custom mansions into the new MM folder, and don't worry about your saves and stuff, it's all in the prefs and application support folders...
Secrets question:
Definition: In the mansion builder, there is an interesting big blue
S
at the end of the sprites that doesn't show up in the game. Now when I started building mansions I wondered what that was... I'll tell you.
A secret is, when you find money that was hidden and there wasn't any obvious way to get to it. (An obvious way is like there's a visible passage, or a door leading to it) The total number of secrets are counted in Percent at the end.
You can also press Command+D in the game, and all these numbers will appear in the corners, the one in the bottom right corner is the number of secrets you've found! The other ones are just room number and framerate. Look in the documentions of the game to find out more about them.
Wingy