How to add custom text settings to a custom theme?
Right now this option in your theme is in configuration setting. I think they done it using some custom module. Edit the module file, add a new text area using following code..

 $form['name']= array(
'#type' =&gt; 'textarea',
'#title' =&gt; t('Main Address'),


);

Save the

$form_state['values']['name']


using variable_set('name' , value) in form submit. Then retrieve variable in template using variable_get('name').

Another easy method is just edit your theme-settings.php. Add a new textarea field. you will get the field in your Home » Administration » Appearance » Settings.
 You can access that value using 

theme_get_setting('field name'); 


Hope this will helps you...