How can I customize the form fields, and front-end templates?

All locator front-end templates files are located in 

components/com_locator/views/

For the search screens, you can manage files located under

components/com_locator/views/directory/tmpl

There you'll find template files that correspond to the available views and also individual files for each type of search control.

The three available views (Combined, Directory Search and Google Map) files are highlighted here:

http://screencast.com/t/pzsJk1BqNpe

Those highlighted view files incorporate the other files based on your settings.

If you are making changes to these files, be sure to follow the Template Override guidlines by making copies first.