moment22, if you use Safari and want to import/export files on your computer, you should open its Preferences, select Security > Website Settings for Internet Plugins > Java and choose Unsafe mode for www.sweethome3d.com web site. But from the attempt I just tried, Safari won't let me select Unsafe mode anymore.
If it's not possible for you too, just use Firefox!
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Emmanuel Puybaret, Sweet Home 3D creator
