I didn't wanna mess with the generic admin folder name. So what i did was to add a new A record to my DNS, say admin.yourdomain.com and then add a directive to your apache config to redirect http://admin.yourdomain.com to http://www.yourdomain.com/youradminfolder something like:
RewriteCond %{HTTP_HOST} =admin.yourdomain.com RewriteRule ^(.*) http://www.yourdomain.com/youadminfolder [R]
I do realise that it is an old post but it was the first hit on google when i searched "prestashop rename admin folder" so i guess someone else could also be helped by having this option.