You don't use PDFCreator by clicking on the icon or starting the program.  If you go into any of your existing applications (like Word, or something) that you can print from (to a normal printer, like you would), then call up a file and go as if you were about to print it normally.  Then look at your list of available printers.  You will find "PDFCreator" has been added to the list.  Pick that.  Now "print" to it like it was a regular printer.  This will start off PDFCreator and you will be invited to Save your PDF off.