Okay, I might have been too vague. As of MBP 1.50 RC1, the game, on Macs, now saves its data in the Application bundle (the .app) itself. I think the question here is where the data files are located? If so, go open "/Users/<your username>/Applications/Marble Blast Platinum.app/platinum/data/". Note the Applications folder is not the system-wide one you may be used to.
Alternatively, in
Finder, press command-shift-G and paste ~/Applications/Marble Blast Platinum.app/platinum/data/ into the dialog box. That should direct you to the folder.
Interiors go in an "interiors" folder, missions go in a "missions" folder. Both folders may or may not be created; if so, you can safely make new folders yourself. To view your preferences. you can press command-uparrow to open the containing folder, with the familiar "client" folder, along with folders such as "server" and "multiplayer". You won't find PlatinumQuest in this, though.
That should be more assistive! If not, please ask for further clarification!
~ PM
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