Problems installing plugins on local WordPress installation

Hi,
I have installed WordPress locally in Ubuntu 13.04 following these instructions https://help.ubuntu.com/community/WordPress.
I am now trying to install a plugin (wordpress-importer but that’s no the point). I have tried direct installation or via upload, everytime I end up on the page asking me for the ftp server details.
I have figured out so far that this is a permission issue, however I have tried everything I came across on the forums without success, i.e.:

sudo chown -R www-data wp-content
sudo chmod -R 755 wp-content

I have also added the following line to wp-config.php:

define('FS_METHOD', 'direct');

I have tried copying and pasting the plugin folder in the plugin directory but it still doesn’t appear in the list of available plugins.

I feel that I’ve run out of options…

Thanks!