The version 4.1.1 of Sweet Home 3D available at the Mac App Store seems to be unable to work under Mac OS X Snow Leopard 10.6 with some configurations, and I'll try to post a fix soon.
Meanwhile, you should request a refund from the App Store, for example by following this help page.
For your information, the free version available on this web site works under Mac OS X Snow Leopard, once you downloaded Java when prompted.