If your trying to Install new themes or a new module and you don't have a Ftp Server installed you'll probably see this warning: "WARNING: You are not using an encrypted connection, so your password will be sent in plain text".
How to solve it quickly in linux:
Open a terminal and copy-past this:
sudo apt-get install vsftpd

Now that you have a ftp server running in your machine, uncomment the following lines ( = delete # from the lines):
local_enable=YES
write_enable=YES
present in the file vsftpd.conf:
- open the file in your favorite editor: nano, kate, gedit...:  
sudo nano /etc/vsftpd.conf

Save the changes in vsftpd.conf and restart the server:
sudo service vsftpd restart

Now, you can insert your linux username and password to fill the forms.

Solution found in: http://drupalalsur.org/apuntes/solucion-al-warning-you-are-not-using-encrypted-connection
My current linux distro: kubuntu 13.10.