The simplest way is the following:
java org.w3c.css.css.StyleSheetCom <your-file-or-URI>
If you launch the validator with no arguments, it gives you all available arguments on command line. Here is some examples :
java org.w3c.css.css.StyleSheetCom http://www.w3.org/ or java org.w3c.css.css.StyleSheetCom http://style.verso.com/stylebot.html?family=1&doc=url4
The servlet class is org.w3c.css.servlet.CssValidator. If you are looking to use Jigsaw, see the ServletWrapper documentation.