Maybe you were working all the time on an automatically recovered file. As such files are not saved at the same place in the free and Apple Store versions, it wasn’t found… yet.
In the Finder, select Go > Go to… menu item, enter ~/Library/Application Support/eTeks/Sweet Home 3D/recovery (tilde sign included) and copy/paste the file you’ll find there to another place. Change its extension from [font=courier new]recovered[/font] to [font=courier new]sh3d[/font] and try to open it.