cannot access wp-admin after installing new theme

Hello, I have installed a new theme and I cannot access wp-admin. It gives me the error message

Not Found

The requested URL /login/ was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I tried to deactivate the theme from filezilla and I was able to access the admin page again.

The WordPress support says

“to find out what caused problem in the theme, upload and run this code when the theme is active:
<? ini_set(‘display_errors’,’1′); ini_set(‘display_startup_errors’,’1′); error_reporting (E_ALL);include(‘index.php’); ?>”

I do not know where I have to upload the above code and how to run it…

Thanks to anyone who wants advice…

Sergio