@sarah

When you unzip the thematic theme you will see a folder called "thematic" this folder is them placed into the themes folder within the wordpress folder structure: [your wordpress folder]/wp-content/themes/.

Within the 'thematic' folder is a folder called 'thematicsamplechildtheme', move/copy this folder up one level within the folder structure (ie into [your wordpress folder]/wp-content/themes/), so both 'thematic' and 'thematicsamplechildtheme' are in the same folder sat alongside all the other themes.

Now when you login to WP as an Admin and click on the Themes section you will see two new themes added. You can them change the css by editing the style.css within the 'thematicsamplechildtheme'.

You will always edit your child themes and leave 'thematic' untouched. This way when ever 'thematic' is updated your child themes will automatically be updated without overwriting any changes you have made.