If you've followed previous instructions you copied StarterKit into it's own folder and made your own sub theme (e.g. horizontal_menu)
Now you can add to themename.info , look for the regions section (ie around line 60)
regions[left_background_column] = left background column
Next copy Zen's page.tpl.php to your sub-theme's directory and add the new region "variable"
You have to look through page.tpl.php for the exact place you want it to appear... OF COURSE, THIS MEANS CREATING SOME CSS TO PLACE IT IN THE PAGE!