Oh. At last I found it! Many Thanks.

Sorry. I've tried to follow Puybaret's instructions. Yet my sweethome is a non-installed version. I run the SweetHome3D-6.4-macosx.dmg directly from the downloaded folder. I can't find C:\Documents and Settings\user\Application Data\eTeks\Sweet Home 3D.

While I was editing a file, the program hanged. Fine. I tried to reopen Sweet Home. Sweet Home automatically open that file (the [recovered] version of the file). Again, it failed to open. I think the recovered version has been corrupted. The problem is every time I reopen SweetHome, it tried again ...