On my foot.php I have tried to show it through a lot of different ways, but I have no idea how elgg actually thinks

$setting_name = 'testpluginid';
$plugin_id = 'testplugin';

echo elgg_get_plugin_setting($setting_name, $plugin_id);

Docs

I don't even know where I have stored the data that had to be given (testpluginid).

Administration -> Plugins -> Your_plugin -> Click on dropdown menu -> Inspect this entity

Yooo! This helped so much! I don't know how to "know where to look for", but you helped me! Even with that Docs link! Thanks man!

And a small extra note; it's all case sensitive.  

You're welcome!

For a quick start to build a plugin: try My plugin