Help a newbie make the location module do something very simple: geocode a zipcode into lat and longitude

Hi I am new to the location module, and I cannot find any documentation to accomplish this seemingly simple task. How do I get Location to geocode a zipcode a user enters, ie transform it into latitude/longitude?

I don't want to use Google Maps, I installed the zipcode table correctly into my database. On the node, I've got the zipcode field showing, it's just when it saves, it doesn't geocode into lat/long. What step am I missing? Which tickbox am I not ticking? Cheers