Moving a copy the thematicsamplechildtheme to wp-content/themes/ and activating it in the Admin > Appearance > Themes area of the WP Admin should work. It does for me.
If you continue to have problems try ftp'ing a new fresh copy of the child theme. If you still see that error message, try zipping the childtheme and using the builtin WordPress theme uploader found at Admin > Appearance > Themes > Install Themes.
There's no need to copy the contents of the default.css in tho the thematicsamplechildtheme's style.css since it is @imported in the file. You may have reasons for doing that but try activating the theme without making any edits to the style.css first.
Also the version of the Thematic Sample Child theme is supposed to be 1.0
ON a side note the sample child theme was just updated in Thematic 1.3.2 which has nott passed trhough theme review yet. There are some signifigant changes to how the custom header image is handled. If you want to experiment with it you can that version here: