Emilie did such a great job of explaining how it works, I've just pasted below her directions.
Just enter the info for your wedding on their site and then give your guests the phone number: DIR-ECT-IONS (347-328-4667) and the name of the event. Then all they have to do is call the number, give the name of the event and where they are driving from - a text message is sent to them with directions. It is free on both sides - you don't pay to post events and your guests don't pay to get the info unless they pay per text message.
Seriously, how can you not LOVE modern technology? Thanks for the great find Emilie.