If you want to change the default text value of the search bar in thesis theme then do the following steps:

1) Look for custom_functions.php file in the thesis theme folder

2) Add this codein  custom_functions.php file to modify the search box text:

function custom_search_text($field_value){
$field_value = ‘Enter Your New Text Here’; return $field_value;
}
add_filter(‘thesis_search_form_value’, ‘custom_search_text’);

You just have to replace this text “Enter Your New Text Here” with whatever text you want to display in the search bar.

NOTE : This is the best way to edit the text of the search bar because if you edit the WordPress default search bar text (here : /wp-content/themes/thesis/lib/widgets.php , in line 30)  then upgrading the WordPress will wipe out your customization.