you need to add in to scope the global variable $imStore example:

function my_custom_function(){
global $imStore;
echo $imStore->store_subnav();
}

or in your template:

 global $imStore;
echo $imStore->store_subnav();

the best option to keep you changes is to create a custom module or add the functions in the theme’s functions.php file by using the plugin’s hooks