I'm using Ubuntu 13.04 and trying to enter this command
sudo add-apt-repository ppa:cairo-dock-team/ppa, but then it asks for my password and I can't type anything. The only thing I can do is press enter, and then it says incorrect password. Please help!
If you entered this command in the terminal, you just enter your password. It won't indicate that it receives anything but validates it when you press RETURN
If you have graphical UI you can use
gksudo instead and it will prompt you for password in a more graphical manner. Try this:
gksudo add-apt-repository ppa:cairo-dock-team/ppa