You have to log in as root to install mplayer.
When you boot up YDL, when it asks for Username, enter in the word
root and then enter your root password that you created when you installed Linux.
If you are already logged in, then there are three ways to become root: (type one of these
bolded commands in a terminal window - the black icon in the menubar)
1.
sudo - allows you to run a command as root, but you must be in a speciall list file over users allowed to use sudo (iirc, /etc/sudoers). you can configure it to allow you to run somethings as root without having to enter the root password or your own account password when running something like "sudo somecommand", and you can make it ask for the root password.
2.
su asks you for root password - always - and when run as "su" in a terminal you'll become root untill you type "exit". You can run it as "su -c 'command'" if you only want to run a single command as root, though a properly setup sudo is probably better if you only intend to run a few commands as root.
su - as above, but the hyphen "-" also loads the root users enviroment, so eg "~" no longer points to you home dir, but to /root. the $PATH also includes the sbin dirs.
I would go with option 2 above. Open up at terminal window and type in
su, and it will ask you for your root password. If you can't remember your root password, you are going to have to reinstall Linux.
Make sure you follow the directions in the How To Install Mplayer thread, read the entire thread before proceeding.
Let me know how it goes.